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Network Working Group B. Aboba
Request for Comments: 3748 Microsoft
Obsoletes: 2284 L. Blunk
Category: Standards Track Merit Network, Inc
J. Vollbrecht
Vollbrecht Consulting LLC
J. Carlson
Sun
H. Levkowetz, Ed.
ipUnplugged
June 2004
Extensible Authentication Protocol (EAP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines the Extensible Authentication Protocol (EAP),
an authentication framework which supports multiple authentication
methods. EAP typically runs directly over data link layers such as
Point-to-Point Protocol (PPP) or IEEE 802, without requiring IP. EAP
provides its own support for duplicate elimination and
retransmission, but is reliant on lower layer ordering guarantees.
Fragmentation is not supported within EAP itself; however, individual
EAP methods may support this.
This document obsoletes RFC 2284. A summary of the changes between
this document and RFC 2284 is available in Appendix A.
Aboba, et al. Standards Track [Page 1]
RFC 3748 EAP June 2004
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Specification of Requirements . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Applicability . . . . . . . . . . . . . . . . . . . . . 6
2. Extensible Authentication Protocol (EAP). . . . . . . . . . . 7
2.1. Support for Sequences . . . . . . . . . . . . . . . . . 9
2.2. EAP Multiplexing Model. . . . . . . . . . . . . . . . . 10
2.3. Pass-Through Behavior . . . . . . . . . . . . . . . . . 12
2.4. Peer-to-Peer Operation. . . . . . . . . . . . . . . . . 14
3. Lower Layer Behavior. . . . . . . . . . . . . . . . . . . . . 15
3.1. Lower Layer Requirements. . . . . . . . . . . . . . . . 15
3.2. EAP Usage Within PPP. . . . . . . . . . . . . . . . . . 18
3.2.1. PPP Configuration Option Format. . . . . . . . . 18
3.3. EAP Usage Within IEEE 802 . . . . . . . . . . . . . . . 19
3.4. Lower Layer Indications . . . . . . . . . . . . . . . . 19
4. EAP Packet Format . . . . . . . . . . . . . . . . . . . . . . 20
4.1. Request and Response. . . . . . . . . . . . . . . . . . 21
4.2. Success and Failure . . . . . . . . . . . . . . . . . . 23
4.3. Retransmission Behavior . . . . . . . . . . . . . . . . 26
5. Initial EAP Request/Response Types. . . . . . . . . . . . . . 27
5.1. Identity. . . . . . . . . . . . . . . . . . . . . . . . 28
5.2. Notification. . . . . . . . . . . . . . . . . . . . . . 29
5.3. Nak . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.3.1. Legacy Nak . . . . . . . . . . . . . . . . . . . 31
5.3.2. Expanded Nak . . . . . . . . . . . . . . . . . . 32
5.4. MD5-Challenge . . . . . . . . . . . . . . . . . . . . . 35
5.5. One-Time Password (OTP) . . . . . . . . . . . . . . . . 36
5.6. Generic Token Card (GTC). . . . . . . . . . . . . . . . 37
5.7. Expanded Types. . . . . . . . . . . . . . . . . . . . . 38
5.8. Experimental. . . . . . . . . . . . . . . . . . . . . . 40
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
6.1. Packet Codes. . . . . . . . . . . . . . . . . . . . . . 41
6.2. Method Types. . . . . . . . . . . . . . . . . . . . . . 41
7. Security Considerations . . . . . . . . . . . . . . . . . . . 42
7.1. Threat Model. . . . . . . . . . . . . . . . . . . . . . 42
7.2. Security Claims . . . . . . . . . . . . . . . . . . . . 43
7.2.1. Security Claims Terminology for EAP Methods. . . 44
7.3. Identity Protection . . . . . . . . . . . . . . . . . . 46
7.4. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . 47
7.5. Packet Modification Attacks . . . . . . . . . . . . . . 48
7.6. Dictionary Attacks. . . . . . . . . . . . . . . . . . . 49
7.7. Connection to an Untrusted Network. . . . . . . . . . . 49
7.8. Negotiation Attacks . . . . . . . . . . . . . . . . . . 50
7.9. Implementation Idiosyncrasies . . . . . . . . . . . . . 50
7.10. Key Derivation. . . . . . . . . . . . . . . . . . . . . 51
7.11. Weak Ciphersuites . . . . . . . . . . . . . . . . . . . 53
Aboba, et al. Standards Track [Page 2]
RFC 3748 EAP June 2004
7.12. Link Layer. . . . . . . . . . . . . . . . . . . . . . . 53
7.13. Separation of Authenticator and Backend Authentication
Server. . . . . . . . . . . . . . . . . . . . . . . . . 54
7.14. Cleartext Passwords . . . . . . . . . . . . . . . . . . 55
7.15. Channel Binding . . . . . . . . . . . . . . . . . . . . 55
7.16. Protected Result Indications. . . . . . . . . . . . . . 56
8. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 58
9. References. . . . . . . . . . . . . . . . . . . . . . . . . . 59
9.1. Normative References. . . . . . . . . . . . . . . . . . 59
9.2. Informative References. . . . . . . . . . . . . . . . . 60
Appendix A. Changes from RFC 2284. . . . . . . . . . . . . . . . . 64
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 66
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 67
1. Introduction
This document defines the Extensible Authentication Protocol (EAP),
an authentication framework which supports multiple authentication
methods. EAP typically runs directly over data link layers such as
Point-to-Point Protocol (PPP) or IEEE 802, without requiring IP. EAP
provides its own support for duplicate elimination and
retransmission, but is reliant on lower layer ordering guarantees.
Fragmentation is not supported within EAP itself; however, individual
EAP methods may support this.
EAP may be used on dedicated links, as well as switched circuits, and
wired as well as wireless links. To date, EAP has been implemented
with hosts and routers that connect via switched circuits or dial-up
lines using PPP [RFC1661]. It has also been implemented with
switches and access points using IEEE 802 [IEEE-802]. EAP
encapsulation on IEEE 802 wired media is described in [IEEE-802.1X],
and encapsulation on IEEE wireless LANs in [IEEE-802.11i].
One of the advantages of the EAP architecture is its flexibility.
EAP is used to select a specific authentication mechanism, typically
after the authenticator requests more information in order to
determine the specific authentication method to be used. Rather than
requiring the authenticator to be updated to support each new
authentication method, EAP permits the use of a backend
authentication server, which may implement some or all authentication
methods, with the authenticator acting as a pass-through for some or
all methods and peers.
Within this document, authenticator requirements apply regardless of
whether the authenticator is operating as a pass-through or not.
Where the requirement is meant to apply to either the authenticator
or backend authentication server, depending on where the EAP
authentication is terminated, the term "EAP server" will be used.
Aboba, et al. Standards Track [Page 3]
RFC 3748 EAP June 2004
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
1.2. Terminology
This document frequently uses the following terms:
authenticator
The end of the link initiating EAP authentication. The term
authenticator is used in [IEEE-802.1X], and has the same meaning
in this document.
peer
The end of the link that responds to the authenticator. In
[IEEE-802.1X], this end is known as the Supplicant.
Supplicant
The end of the link that responds to the authenticator in [IEEE-
802.1X]. In this document, this end of the link is called the
peer.
backend authentication server
A backend authentication server is an entity that provides an
authentication service to an authenticator. When used, this
server typically executes EAP methods for the authenticator. This
terminology is also used in [IEEE-802.1X].
AAA
Authentication, Authorization, and Accounting. AAA protocols with
EAP support include RADIUS [RFC3579] and Diameter [DIAM-EAP]. In
this document, the terms "AAA server" and "backend authentication
server" are used interchangeably.
Displayable Message
This is interpreted to be a human readable string of characters.
The message encoding MUST follow the UTF-8 transformation format
[RFC2279].
Aboba, et al. Standards Track [Page 4]
RFC 3748 EAP June 2004
EAP server
The entity that terminates the EAP authentication method with the
peer. In the case where no backend authentication server is used,
the EAP server is part of the authenticator. In the case where
the authenticator operates in pass-through mode, the EAP server is
located on the backend authentication server.
Silently Discard
This means the implementation discards the packet without further
processing. The implementation SHOULD provide the capability of
logging the event, including the contents of the silently
discarded packet, and SHOULD record the event in a statistics
counter.
Successful Authentication
In the context of this document, "successful authentication" is an
exchange of EAP messages, as a result of which the authenticator
decides to allow access by the peer, and the peer decides to use
this access. The authenticator's decision typically involves both
authentication and authorization aspects; the peer may
successfully authenticate to the authenticator, but access may be
denied by the authenticator due to policy reasons.
Message Integrity Check (MIC)
A keyed hash function used for authentication and integrity
protection of data. This is usually called a Message
Authentication Code (MAC), but IEEE 802 specifications (and this
document) use the acronym MIC to avoid confusion with Medium
Access Control.
Cryptographic Separation
Two keys (x and y) are "cryptographically separate" if an
adversary that knows all messages exchanged in the protocol cannot
compute x from y or y from x without "breaking" some cryptographic
assumption. In particular, this definition allows that the
adversary has the knowledge of all nonces sent in cleartext, as
well as all predictable counter values used in the protocol.
Breaking a cryptographic assumption would typically require
inverting a one-way function or predicting the outcome of a
cryptographic pseudo-random number generator without knowledge of
the secret state. In other words, if the keys are
cryptographically separate, there is no shortcut to compute x from
y or y from x, but the work an adversary must do to perform this
computation is equivalent to performing an exhaustive search for
the secret state value.
Aboba, et al. Standards Track [Page 5]
RFC 3748 EAP June 2004
Master Session Key (MSK)
Keying material that is derived between the EAP peer and server
and exported by the EAP method. The MSK is at least 64 octets in
length. In existing implementations, a AAA server acting as an
EAP server transports the MSK to the authenticator.
Extended Master Session Key (EMSK)
Additional keying material derived between the EAP client and
server that is exported by the EAP method. The EMSK is at least
64 octets in length. The EMSK is not shared with the
authenticator or any other third party. The EMSK is reserved for
future uses that are not defined yet.
Result indications
A method provides result indications if after the method's last
message is sent and received:
1) The peer is aware of whether it has authenticated the server,
as well as whether the server has authenticated it.
2) The server is aware of whether it has authenticated the peer,
as well as whether the peer has authenticated it.
In the case where successful authentication is sufficient to
authorize access, then the peer and authenticator will also know if
the other party is willing to provide or accept access. This may not
always be the case. An authenticated peer may be denied access due
to lack of authorization (e.g., session limit) or other reasons.
Since the EAP exchange is run between the peer and the server, other
nodes (such as AAA proxies) may also affect the authorization
decision. This is discussed in more detail in Section 7.16.
1.3. Applicability
EAP was designed for use in network access authentication, where IP
layer connectivity may not be available. Use of EAP for other
purposes, such as bulk data transport, is NOT RECOMMENDED.
Since EAP does not require IP connectivity, it provides just enough
support for the reliable transport of authentication protocols, and
no more.
EAP is a lock-step protocol which only supports a single packet in
flight. As a result, EAP cannot efficiently transport bulk data,
unlike transport protocols such as TCP [RFC793] or SCTP [RFC2960].
Aboba, et al. Standards Track [Page 6]
RFC 3748 EAP June 2004
While EAP provides support for retransmission, it assumes ordering
guarantees provided by the lower layer, so out of order reception is
not supported.
Since EAP does not support fragmentation and reassembly, EAP
authentication methods generating payloads larger than the minimum
EAP MTU need to provide fragmentation support.
While authentication methods such as EAP-TLS [RFC2716] provide
support for fragmentation and reassembly, the EAP methods defined in
this document do not. As a result, if the EAP packet size exceeds
the EAP MTU of the link, these methods will encounter difficulties.
EAP authentication is initiated by the server (authenticator),
whereas many authentication protocols are initiated by the client
(peer). As a result, it may be necessary for an authentication
algorithm to add one or two additional messages (at most one
roundtrip) in order to run over EAP.
Where certificate-based authentication is supported, the number of
additional roundtrips may be much larger due to fragmentation of
certificate chains. In general, a fragmented EAP packet will require
as many round-trips to send as there are fragments. For example, a
certificate chain 14960 octets in size would require ten round-trips
to send with a 1496 octet EAP MTU.
Where EAP runs over a lower layer in which significant packet loss is
experienced, or where the connection between the authenticator and
authentication server experiences significant packet loss, EAP
methods requiring many round-trips can experience difficulties. In
these situations, use of EAP methods with fewer roundtrips is
advisable.
2. Extensible Authentication Protocol (EAP)
The EAP authentication exchange proceeds as follows:
[1] The authenticator sends a Request to authenticate the peer. The
Request has a Type field to indicate what is being requested.
Examples of Request Types include Identity, MD5-challenge, etc.
The MD5-challenge Type corresponds closely to the CHAP
authentication protocol [RFC1994]. Typically, the authenticator
will send an initial Identity Request; however, an initial
Identity Request is not required, and MAY be bypassed. For
example, the identity may not be required where it is determined
by the port to which the peer has connected (leased lines,
Aboba, et al. Standards Track [Page 7]
RFC 3748 EAP June 2004
dedicated switch or dial-up ports), or where the identity is
obtained in another fashion (via calling station identity or MAC
address, in the Name field of the MD5-Challenge Response, etc.).
[2] The peer sends a Response packet in reply to a valid Request. As
with the Request packet, the Response packet contains a Type
field, which corresponds to the Type field of the Request.
[3] The authenticator sends an additional Request packet, and the
peer replies with a Response. The sequence of Requests and
Responses continues as long as needed. EAP is a 'lock step'
protocol, so that other than the initial Request, a new Request
cannot be sent prior to receiving a valid Response. The
authenticator is responsible for retransmitting requests as
described in Section 4.1. After a suitable number of
retransmissions, the authenticator SHOULD end the EAP
conversation. The authenticator MUST NOT send a Success or
Failure packet when retransmitting or when it fails to get a
response from the peer.
[4] The conversation continues until the authenticator cannot
authenticate the peer (unacceptable Responses to one or more
Requests), in which case the authenticator implementation MUST
transmit an EAP Failure (Code 4). Alternatively, the
authentication conversation can continue until the authenticator
determines that successful authentication has occurred, in which
case the authenticator MUST transmit an EAP Success (Code 3).
Advantages:
o The EAP protocol can support multiple authentication mechanisms
without having to pre-negotiate a particular one.
o Network Access Server (NAS) devices (e.g., a switch or access
point) do not have to understand each authentication method and
MAY act as a pass-through agent for a backend authentication
server. Support for pass-through is optional. An authenticator
MAY authenticate local peers, while at the same time acting as a
pass-through for non-local peers and authentication methods it
does not implement locally.
o Separation of the authenticator from the backend authentication
server simplifies credentials management and policy decision
making.
Aboba, et al. Standards Track [Page 8]
RFC 3748 EAP June 2004
Disadvantages:
o For use in PPP, EAP requires the addition of a new authentication
Type to PPP LCP and thus PPP implementations will need to be
modified to use it. It also strays from the previous PPP
authentication model of negotiating a specific authentication
mechanism during LCP. Similarly, switch or access point
implementations need to support [IEEE-802.1X] in order to use EAP.
o Where the authenticator is separate from the backend
authentication server, this complicates the security analysis and,
if needed, key distribution.
2.1. Support for Sequences
An EAP conversation MAY utilize a sequence of methods. A common
example of this is an Identity request followed by a single EAP
authentication method such as an MD5-Challenge. However, the peer
and authenticator MUST utilize only one authentication method (Type 4
or greater) within an EAP conversation, after which the authenticator
MUST send a Success or Failure packet.
Once a peer has sent a Response of the same Type as the initial
Request, an authenticator MUST NOT send a Request of a different Type
prior to completion of the final round of a given method (with the
exception of a Notification-Request) and MUST NOT send a Request for
an additional method of any Type after completion of the initial
authentication method; a peer receiving such Requests MUST treat them
as invalid, and silently discard them. As a result, Identity Requery
is not supported.
A peer MUST NOT send a Nak (legacy or expanded) in reply to a Request
after an initial non-Nak Response has been sent. Since spoofed EAP
Request packets may be sent by an attacker, an authenticator
receiving an unexpected Nak SHOULD discard it and log the event.
Multiple authentication methods within an EAP conversation are not
supported due to their vulnerability to man-in-the-middle attacks
(see Section 7.4) and incompatibility with existing implementations.
Where a single EAP authentication method is utilized, but other
methods are run within it (a "tunneled" method), the prohibition
against multiple authentication methods does not apply. Such
"tunneled" methods appear as a single authentication method to EAP.
Backward compatibility can be provided, since a peer not supporting a
"tunneled" method can reply to the initial EAP-Request with a Nak
Aboba, et al. Standards Track [Page 9]
RFC 3748 EAP June 2004
(legacy or expanded). To address security vulnerabilities,
"tunneled" methods MUST support protection against man-in-the-middle
attacks.
2.2. EAP Multiplexing Model
Conceptually, EAP implementations consist of the following
components:
[a] Lower layer. The lower layer is responsible for transmitting and
receiving EAP frames between the peer and authenticator. EAP has
been run over a variety of lower layers including PPP, wired IEEE
802 LANs [IEEE-802.1X], IEEE 802.11 wireless LANs [IEEE-802.11],
UDP (L2TP [RFC2661] and IKEv2 [IKEv2]), and TCP [PIC]. Lower
layer behavior is discussed in Section 3.
[b] EAP layer. The EAP layer receives and transmits EAP packets via
the lower layer, implements duplicate detection and
retransmission, and delivers and receives EAP messages to and
from the EAP peer and authenticator layers.
[c] EAP peer and authenticator layers. Based on the Code field, the
EAP layer demultiplexes incoming EAP packets to the EAP peer and
authenticator layers. Typically, an EAP implementation on a
given host will support either peer or authenticator
functionality, but it is possible for a host to act as both an
EAP peer and authenticator. In such an implementation both EAP
peer and authenticator layers will be present.
[d] EAP method layers. EAP methods implement the authentication
algorithms and receive and transmit EAP messages via the EAP peer
and authenticator layers. Since fragmentation support is not
provided by EAP itself, this is the responsibility of EAP
methods, which are discussed in Section 5.
The EAP multiplexing model is illustrated in Figure 1 below. Note
that there is no requirement that an implementation conform to this
model, as long as the on-the-wire behavior is consistent with it.
Aboba, et al. Standards Track [Page 10]
RFC 3748 EAP June 2004
+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |
| EAP method| EAP method| | EAP method| EAP method|
| Type = X | Type = Y | | Type = X | Type = Y |
| V | | | ^ | |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
| ! | | ! |
| EAP ! Peer layer | | EAP ! Auth. layer |
| ! | | ! |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
| ! | | ! |
| EAP ! layer | | EAP ! layer |
| ! | | ! |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
| ! | | ! |
| Lower ! layer | | Lower ! layer |
| ! | | ! |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
! !
! Peer ! Authenticator
+------------>-------------+
Figure 1: EAP Multiplexing Model
Within EAP, the Code field functions much like a protocol number in
IP. It is assumed that the EAP layer demultiplexes incoming EAP
packets according to the Code field. Received EAP packets with
Code=1 (Request), 3 (Success), and 4 (Failure) are delivered by the
EAP layer to the EAP peer layer, if implemented. EAP packets with
Code=2 (Response) are delivered to the EAP authenticator layer, if
implemented.
Within EAP, the Type field functions much like a port number in UDP
or TCP. It is assumed that the EAP peer and authenticator layers
demultiplex incoming EAP packets according to their Type, and deliver
them only to the EAP method corresponding to that Type. An EAP
method implementation on a host may register to receive packets from
the peer or authenticator layers, or both, depending on which role(s)
it supports.
Since EAP authentication methods may wish to access the Identity,
implementations SHOULD make the Identity Request and Response
accessible to authentication methods (Types 4 or greater), in
addition to the Identity method. The Identity Type is discussed in
Section 5.1.
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RFC 3748 EAP June 2004
A Notification Response is only used as confirmation that the peer
received the Notification Request, not that it has processed it, or
displayed the message to the user. It cannot be assumed that the
contents of the Notification Request or Response are available to
another method. The Notification Type is discussed in Section 5.2.
Nak (Type 3) or Expanded Nak (Type 254) are utilized for the purposes
of method negotiation. Peers respond to an initial EAP Request for
an unacceptable Type with a Nak Response (Type 3) or Expanded Nak
Response (Type 254). It cannot be assumed that the contents of the
Nak Response(s) are available to another method. The Nak Type(s) are
discussed in Section 5.3.
EAP packets with Codes of Success or Failure do not include a Type
field, and are not delivered to an EAP method. Success and Failure
are discussed in Section 4.2.
Given these considerations, the Success, Failure, Nak Response(s),
and Notification Request/Response messages MUST NOT be used to carry
data destined for delivery to other EAP methods.
2.3. Pass-Through Behavior
When operating as a "pass-through authenticator", an authenticator
performs checks on the Code, Identifier, and Length fields as
described in Section 4.1. It forwards EAP packets received from the
peer and destined to its authenticator layer to the backend
authentication server; packets received from the backend
authentication server destined to the peer are forwarded to it.
A host receiving an EAP packet may only do one of three things with
it: act on it, drop it, or forward it. The forwarding decision is
typically based only on examination of the Code, Identifier, and
Length fields. A pass-through authenticator implementation MUST be
capable of forwarding EAP packets received from the peer with Code=2
(Response) to the backend authentication server. It also MUST be
capable of receiving EAP packets from the backend authentication
server and forwarding EAP packets of Code=1 (Request), Code=3
(Success), and Code=4 (Failure) to the peer.
Unless the authenticator implements one or more authentication
methods locally which support the authenticator role, the EAP method
layer header fields (Type, Type-Data) are not examined as part of the
forwarding decision. Where the authenticator supports local
authentication methods, it MAY examine the Type field to determine
whether to act on the packet itself or forward it. Compliant pass-
through authenticator implementations MUST by default forward EAP
packets of any Type.
Aboba, et al. Standards Track [Page 12]
RFC 3748 EAP June 2004
EAP packets received with Code=1 (Request), Code=3 (Success), and
Code=4 (Failure) are demultiplexed by the EAP layer and delivered to
the peer layer. Therefore, unless a host implements an EAP peer
layer, these packets will be silently discarded. Similarly, EAP
packets received with Code=2 (Response) are demultiplexed by the EAP
layer and delivered to the authenticator layer. Therefore, unless a
host implements an EAP authenticator layer, these packets will be
silently discarded. The behavior of a "pass-through peer" is
undefined within this specification, and is unsupported by AAA
protocols such as RADIUS [RFC3579] and Diameter [DIAM-EAP].
The forwarding model is illustrated in Figure 2.
Peer Pass-through Authenticator Authentication
Server
+-+-+-+-+-+-+ +-+-+-+-+-+-+
| | | |
|EAP method | |EAP method |
| V | | ^ |
+-+-+-!-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-!-+-+-+
| ! | |EAP | EAP | | | ! |
| ! | |Peer | Auth.| EAP Auth. | | ! |
|EAP ! peer| | | +-----------+ | |EAP !Auth.|
| ! | | | ! | ! | | ! |
+-+-+-!-+-+-+ +-+-+-+-!-+-+-+-+-+-!-+-+-+-+ +-+-+-!-+-+-+
| ! | | ! | ! | | ! |
|EAP !layer| | EAP !layer| EAP !layer | |EAP !layer|
| ! | | ! | ! | | ! |
+-+-+-!-+-+-+ +-+-+-+-!-+-+-+-+-+-!-+-+-+-+ +-+-+-!-+-+-+
| ! | | ! | ! | | ! |
|Lower!layer| | Lower!layer| AAA ! /IP | | AAA ! /IP |
| ! | | ! | ! | | ! |
+-+-+-!-+-+-+ +-+-+-+-!-+-+-+-+-+-!-+-+-+-+ +-+-+-!-+-+-+
! ! ! !
! ! ! !
+-------->--------+ +--------->-------+
Figure 2: Pass-through Authenticator
For sessions in which the authenticator acts as a pass-through, it
MUST determine the outcome of the authentication solely based on the
Accept/Reject indication sent by the backend authentication server;
the outcome MUST NOT be determined by the contents of an EAP packet
sent along with the Accept/Reject indication, or the absence of such
an encapsulated EAP packet.
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2.4. Peer-to-Peer Operation
Since EAP is a peer-to-peer protocol, an independent and simultaneous
authentication may take place in the reverse direction (depending on
the capabilities of the lower layer). Both ends of the link may act
as authenticators and peers at the same time. In this case, it is
necessary for both ends to implement EAP authenticator and peer
layers. In addition, the EAP method implementations on both peers
must support both authenticator and peer functionality.
Although EAP supports peer-to-peer operation, some EAP
implementations, methods, AAA protocols, and link layers may not
support this. Some EAP methods may support asymmetric
authentication, with one type of credential being required for the
peer and another type for the authenticator. Hosts supporting peer-
to-peer operation with such a method would need to be provisioned
with both types of credentials.
For example, EAP-TLS [RFC2716] is a client-server protocol in which
distinct certificate profiles are typically utilized for the client
and server. This implies that a host supporting peer-to-peer
authentication with EAP-TLS would need to implement both the EAP peer
and authenticator layers, support both peer and authenticator roles
in the EAP-TLS implementation, and provision certificates appropriate
for each role.
AAA protocols such as RADIUS/EAP [RFC3579] and Diameter EAP [DIAM-
EAP] only support "pass-through authenticator" operation. As noted
in [RFC3579] Section 2.6.2, a RADIUS server responds to an Access-
Request encapsulating an EAP-Request, Success, or Failure packet with
an Access-Reject. There is therefore no support for "pass-through
peer" operation.
Even where a method is used which supports mutual authentication and
result indications, several considerations may dictate that two EAP
authentications (one in each direction) are required. These include:
[1] Support for bi-directional session key derivation in the lower
layer. Lower layers such as IEEE 802.11 may only support uni-
directional derivation and transport of transient session keys.
For example, the group-key handshake defined in [IEEE-802.11i] is
uni-directional, since in IEEE 802.11 infrastructure mode, only
the Access Point (AP) sends multicast/broadcast traffic. In IEEE
802.11 ad hoc mode, where either peer may send
multicast/broadcast traffic, two uni-directional group-key
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exchanges are required. Due to limitations of the design, this
also implies the need for unicast key derivations and EAP method
exchanges to occur in each direction.
[2] Support for tie-breaking in the lower layer. Lower layers such
as IEEE 802.11 ad hoc do not support "tie breaking" wherein two
hosts initiating authentication with each other will only go
forward with a single authentication. This implies that even if
802.11 were to support a bi-directional group-key handshake, then
two authentications, one in each direction, might still occur.
[3] Peer policy satisfaction. EAP methods may support result
indications, enabling the peer to indicate to the EAP server
within the method that it successfully authenticated the EAP
server, as well as for the server to indicate that it has
authenticated the peer. However, a pass-through authenticator
will not be aware that the peer has accepted the credentials
offered by the EAP server, unless this information is provided to
the authenticator via the AAA protocol. The authenticator SHOULD
interpret the receipt of a key attribute within an Accept packet
as an indication that the peer has successfully authenticated the
server.
However, it is possible that the EAP peer's access policy was not
satisfied during the initial EAP exchange, even though mutual
authentication occurred. For example, the EAP authenticator may not
have demonstrated authorization to act in both peer and authenticator
roles. As a result, the peer may require an additional
authentication in the reverse direction, even if the peer provided an
indication that the EAP server had successfully authenticated to it.
3. Lower Layer Behavior
3.1. Lower Layer Requirements
EAP makes the following assumptions about lower layers:
[1] Unreliable transport. In EAP, the authenticator retransmits
Requests that have not yet received Responses so that EAP does
not assume that lower layers are reliable. Since EAP defines its
own retransmission behavior, it is possible (though undesirable)
for retransmission to occur both in the lower layer and the EAP
layer when EAP is run over a reliable lower layer.
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Note that EAP Success and Failure packets are not retransmitted.
Without a reliable lower layer, and with a non-negligible error rate,
these packets can be lost, resulting in timeouts. It is therefore
desirable for implementations to improve their resilience to loss of
EAP Success or Failure packets, as described in Section 4.2.
[2] Lower layer error detection. While EAP does not assume that the
lower layer is reliable, it does rely on lower layer error
detection (e.g., CRC, Checksum, MIC, etc.). EAP methods may not
include a MIC, or if they do, it may not be computed over all the
fields in the EAP packet, such as the Code, Identifier, Length,
or Type fields. As a result, without lower layer error
detection, undetected errors could creep into the EAP layer or
EAP method layer header fields, resulting in authentication
failures.
For example, EAP TLS [RFC2716], which computes its MIC over the
Type-Data field only, regards MIC validation failures as a fatal
error. Without lower layer error detection, this method, and
others like it, will not perform reliably.
[3] Lower layer security. EAP does not require lower layers to
provide security services such as per-packet confidentiality,
authentication, integrity, and replay protection. However, where
these security services are available, EAP methods supporting Key
Derivation (see Section 7.2.1) can be used to provide dynamic
keying material. This makes it possible to bind the EAP
authentication to subsequent data and protect against data
modification, spoofing, or replay. See Section 7.1 for details.
[4] Minimum MTU. EAP is capable of functioning on lower layers that
provide an EAP MTU size of 1020 octets or greater.
EAP does not support path MTU discovery, and fragmentation and
reassembly is not supported by EAP, nor by the methods defined in
this specification: Identity (1), Notification (2), Nak Response
(3), MD5-Challenge (4), One Time Password (5), Generic Token Card
(6), and expanded Nak Response (254) Types.
Typically, the EAP peer obtains information on the EAP MTU from
the lower layers and sets the EAP frame size to an appropriate
value. Where the authenticator operates in pass-through mode,
the authentication server does not have a direct way of
determining the EAP MTU, and therefore relies on the
authenticator to provide it with this information, such as via
the Framed-MTU attribute, as described in [RFC3579], Section 2.4.
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While methods such as EAP-TLS [RFC2716] support fragmentation and
reassembly, EAP methods originally designed for use within PPP
where a 1500 octet MTU is guaranteed for control frames (see
[RFC1661], Section 6.1) may lack fragmentation and reassembly
features.
EAP methods can assume a minimum EAP MTU of 1020 octets in the
absence of other information. EAP methods SHOULD include support
for fragmentation and reassembly if their payloads can be larger
than this minimum EAP MTU.
EAP is a lock-step protocol, which implies a certain inefficiency
when handling fragmentation and reassembly. Therefore, if the
lower layer supports fragmentation and reassembly (such as where
EAP is transported over IP), it may be preferable for
fragmentation and reassembly to occur in the lower layer rather
than in EAP. This can be accomplished by providing an
artificially large EAP MTU to EAP, causing fragmentation and
reassembly to be handled within the lower layer.
[5] Possible duplication. Where the lower layer is reliable, it will
provide the EAP layer with a non-duplicated stream of packets.
However, while it is desirable that lower layers provide for
non-duplication, this is not a requirement. The Identifier field
provides both the peer and authenticator with the ability to
detect duplicates.
[6] Ordering guarantees. EAP does not require the Identifier to be
monotonically increasing, and so is reliant on lower layer
ordering guarantees for correct operation. EAP was originally
defined to run on PPP, and [RFC1661] Section 1 has an ordering
requirement:
"The Point-to-Point Protocol is designed for simple links
which transport packets between two peers. These links
provide full-duplex simultaneous bi-directional operation,
and are assumed to deliver packets in order."
Lower layer transports for EAP MUST preserve ordering between a
source and destination at a given priority level (the ordering
guarantee provided by [IEEE-802]).
Reordering, if it occurs, will typically result in an EAP
authentication failure, causing EAP authentication to be re-run.
In an environment in which reordering is likely, it is therefore
expected that EAP authentication failures will be common. It is
RECOMMENDED that EAP only be run over lower layers that provide
ordering guarantees; running EAP over raw IP or UDP transport is
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NOT RECOMMENDED. Encapsulation of EAP within RADIUS [RFC3579]
satisfies ordering requirements, since RADIUS is a "lockstep"
protocol that delivers packets in order.
3.2. EAP Usage Within PPP
In order to establish communications over a point-to-point link, each
end of the PPP link first sends LCP packets to configure the data
link during the Link Establishment phase. After the link has been
established, PPP provides for an optional Authentication phase before
proceeding to the Network-Layer Protocol phase.
By default, authentication is not mandatory. If authentication of
the link is desired, an implementation MUST specify the
Authentication Protocol Configuration Option during the Link
Establishment phase.
If the identity of the peer has been established in the
Authentication phase, the server can use that identity in the
selection of options for the following network layer negotiations.
When implemented within PPP, EAP does not select a specific
authentication mechanism at the PPP Link Control Phase, but rather
postpones this until the Authentication Phase. This allows the
authenticator to request more information before determining the
specific authentication mechanism. This also permits the use of a
"backend" server which actually implements the various mechanisms
while the PPP authenticator merely passes through the authentication
exchange. The PPP Link Establishment and Authentication phases, and
the Authentication Protocol Configuration Option, are defined in The
Point-to-Point Protocol (PPP) [RFC1661].
3.2.1. PPP Configuration Option Format
A summary of the PPP Authentication Protocol Configuration Option
format to negotiate EAP follows. The fields are transmitted from
left to right.
Exactly one EAP packet is encapsulated in the Information field of a
PPP Data Link Layer frame where the protocol field indicates type hex
C227 (PPP EAP).
Aboba, et al. Standards Track [Page 18]
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Authentication Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
3
Length
4
Authentication Protocol
C227 (Hex) for Extensible Authentication Protocol (EAP)
3.3. EAP Usage Within IEEE 802
The encapsulation of EAP over IEEE 802 is defined in [IEEE-802.1X].
The IEEE 802 encapsulation of EAP does not involve PPP, and IEEE
802.1X does not include support for link or network layer
negotiations. As a result, within IEEE 802.1X, it is not possible to
negotiate non-EAP authentication mechanisms, such as PAP or CHAP
[RFC1994].
3.4. Lower Layer Indications
The reliability and security of lower layer indications is dependent
on the lower layer. Since EAP is media independent, the presence or
absence of lower layer security is not taken into account in the
processing of EAP messages.
To improve reliability, if a peer receives a lower layer success
indication as defined in Section 7.2, it MAY conclude that a Success
packet has been lost, and behave as if it had actually received a
Success packet. This includes choosing to ignore the Success in some
circumstances as described in Section 4.2.
A discussion of some reliability and security issues with lower layer
indications in PPP, IEEE 802 wired networks, and IEEE 802.11 wireless
LANs can be found in the Security Considerations, Section 7.12.
After EAP authentication is complete, the peer will typically
transmit and receive data via the authenticator. It is desirable to
provide assurance that the entities transmitting data are the same
ones that successfully completed EAP authentication. To accomplish
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this, it is necessary for the lower layer to provide per-packet
integrity, authentication and replay protection, and to bind these
per-packet services to the keys derived during EAP authentication.
Otherwise, it is possible for subsequent data traffic to be modified,
spoofed, or replayed.
Where keying material for the lower layer ciphersuite is itself
provided by EAP, ciphersuite negotiation and key activation are
controlled by the lower layer. In PPP, ciphersuites are negotiated
within ECP so that it is not possible to use keys derived from EAP
authentication until the completion of ECP. Therefore, an initial
EAP exchange cannot be protected by a PPP ciphersuite, although EAP
re-authentication can be protected.
In IEEE 802 media, initial key activation also typically occurs after
completion of EAP authentication. Therefore an initial EAP exchange
typically cannot be protected by the lower layer ciphersuite,
although an EAP re-authentication or pre-authentication exchange can
be protected.
4. EAP Packet Format
A summary of the EAP packet format is shown below. The fields are
transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
The Code field is one octet and identifies the Type of EAP packet.
EAP Codes are assigned as follows:
1 Request
2 Response
3 Success
4 Failure
Since EAP only defines Codes 1-4, EAP packets with other codes
MUST be silently discarded by both authenticators and peers.
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Identifier
The Identifier field is one octet and aids in matching Responses
with Requests.
Length
The Length field is two octets and indicates the length, in
octets, of the EAP packet including the Code, Identifier, Length,
and Data fields. Octets outside the range of the Length field
should be treated as Data Link Layer padding and MUST be ignored
upon reception. A message with the Length field set to a value
larger than the number of received octets MUST be silently
discarded.
Data
The Data field is zero or more octets. The format of the Data
field is determined by the Code field.
4.1. Request and Response
Description
The Request packet (Code field set to 1) is sent by the
authenticator to the peer. Each Request has a Type field which
serves to indicate what is being requested. Additional Request
packets MUST be sent until a valid Response packet is received, an
optional retry counter expires, or a lower layer failure
indication is received.
Retransmitted Requests MUST be sent with the same Identifier value
in order to distinguish them from new Requests. The content of
the data field is dependent on the Request Type. The peer MUST
send a Response packet in reply to a valid Request packet.
Responses MUST only be sent in reply to a valid Request and never
be retransmitted on a timer.
If a peer receives a valid duplicate Request for which it has
already sent a Response, it MUST resend its original Response
without reprocessing the Request. Requests MUST be processed in
the order that they are received, and MUST be processed to their
completion before inspecting the next Request.
A summary of the Request and Response packet format follows. The
fields are transmitted from left to right.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Type-Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
1 for Request
2 for Response
Identifier
The Identifier field is one octet. The Identifier field MUST be
the same if a Request packet is retransmitted due to a timeout
while waiting for a Response. Any new (non-retransmission)
Requests MUST modify the Identifier field.
The Identifier field of the Response MUST match that of the
currently outstanding Request. An authenticator receiving a
Response whose Identifier value does not match that of the
currently outstanding Request MUST silently discard the Response.
In order to avoid confusion between new Requests and
retransmissions, the Identifier value chosen for each new Request
need only be different from the previous Request, but need not be
unique within the conversation. One way to achieve this is to
start the Identifier at an initial value and increment it for each
new Request. Initializing the first Identifier with a random
number rather than starting from zero is recommended, since it
makes sequence attacks somewhat more difficult.
Since the Identifier space is unique to each session,
authenticators are not restricted to only 256 simultaneous
authentication conversations. Similarly, with re-authentication,
an EAP conversation might continue over a long period of time, and
is not limited to only 256 roundtrips.
Implementation Note: The authenticator is responsible for
retransmitting Request messages. If the Request message is obtained
from elsewhere (such as from a backend authentication server), then
the authenticator will need to save a copy of the Request in order to
accomplish this. The peer is responsible for detecting and handling
duplicate Request messages before processing them in any way,
including passing them on to an outside party. The authenticator is
also responsible for discarding Response messages with a non-matching
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Identifier value before acting on them in any way, including passing
them on to the backend authentication server for verification. Since
the authenticator can retransmit before receiving a Response from the
peer, the authenticator can receive multiple Responses, each with a
matching Identifier. Until a new Request is received by the
authenticator, the Identifier value is not updated, so that the
authenticator forwards Responses to the backend authentication
server, one at a time.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and Type-Data
fields. Octets outside the range of the Length field should be
treated as Data Link Layer padding and MUST be ignored upon
reception. A message with the Length field set to a value larger
than the number of received octets MUST be silently discarded.
Type
The Type field is one octet. This field indicates the Type of
Request or Response. A single Type MUST be specified for each EAP
Request or Response. An initial specification of Types follows in
Section 5 of this document.
The Type field of a Response MUST either match that of the
Request, or correspond to a legacy or Expanded Nak (see Section
5.3) indicating that a Request Type is unacceptable to the peer.
A peer MUST NOT send a Nak (legacy or expanded) in response to a
Request, after an initial non-Nak Response has been sent. An EAP
server receiving a Response not meeting these requirements MUST
silently discard it.
Type-Data
The Type-Data field varies with the Type of Request and the
associated Response.
4.2. Success and Failure
The Success packet is sent by the authenticator to the peer after
completion of an EAP authentication method (Type 4 or greater) to
indicate that the peer has authenticated successfully to the
authenticator. The authenticator MUST transmit an EAP packet with
the Code field set to 3 (Success). If the authenticator cannot
authenticate the peer (unacceptable Responses to one or more
Requests), then after unsuccessful completion of the EAP method in
progress, the implementation MUST transmit an EAP packet with the
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Code field set to 4 (Failure). An authenticator MAY wish to issue
multiple Requests before sending a Failure response in order to allow
for human typing mistakes. Success and Failure packets MUST NOT
contain additional data.
Success and Failure packets MUST NOT be sent by an EAP authenticator
if the specification of the given method does not explicitly permit
the method to finish at that point. A peer EAP implementation
receiving a Success or Failure packet where sending one is not
explicitly permitted MUST silently discard it. By default, an EAP
peer MUST silently discard a "canned" Success packet (a Success
packet sent immediately upon connection). This ensures that a rogue
authenticator will not be able to bypass mutual authentication by
sending a Success packet prior to conclusion of the EAP method
conversation.
Implementation Note: Because the Success and Failure packets are not
acknowledged, they are not retransmitted by the authenticator, and
may be potentially lost. A peer MUST allow for this circumstance as
described in this note. See also Section 3.4 for guidance on the
processing of lower layer success and failure indications.
As described in Section 2.1, only a single EAP authentication method
is allowed within an EAP conversation. EAP methods may implement
result indications. After the authenticator sends a failure result
indication to the peer, regardless of the response from the peer, it
MUST subsequently send a Failure packet. After the authenticator
sends a success result indication to the peer and receives a success
result indication from the peer, it MUST subsequently send a Success
packet.
On the peer, once the method completes unsuccessfully (that is,
either the authenticator sends a failure result indication, or the
peer decides that it does not want to continue the conversation,
possibly after sending a failure result indication), the peer MUST
terminate the conversation and indicate failure to the lower layer.
The peer MUST silently discard Success packets and MAY silently
discard Failure packets. As a result, loss of a Failure packet need
not result in a timeout.
On the peer, after success result indications have been exchanged by
both sides, a Failure packet MUST be silently discarded. The peer
MAY, in the event that an EAP Success is not received, conclude that
the EAP Success packet was lost and that authentication concluded
successfully.
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If the authenticator has not sent a result indication, and the peer
is willing to continue the conversation, the peer waits for a Success
or Failure packet once the method completes, and MUST NOT silently
discard either of them. In the event that neither a Success nor
Failure packet is received, the peer SHOULD terminate the
conversation to avoid lengthy timeouts in case the lost packet was an
EAP Failure.
If the peer attempts to authenticate to the authenticator and fails
to do so, the authenticator MUST send a Failure packet and MUST NOT
grant access by sending a Success packet. However, an authenticator
MAY omit having the peer authenticate to it in situations where
limited access is offered (e.g., guest access). In this case, the
authenticator MUST send a Success packet.
Where the peer authenticates successfully to the authenticator, but
the authenticator does not send a result indication, the
authenticator MAY deny access by sending a Failure packet where the
peer is not currently authorized for network access.
A summary of the Success and Failure packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
3 for Success
4 for Failure
Identifier
The Identifier field is one octet and aids in matching replies to
Responses. The Identifier field MUST match the Identifier field
of the Response packet that it is sent in response to.
Length
4
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4.3. Retransmission Behavior
Because the authentication process will often involve user input,
some care must be taken when deciding upon retransmission strategies
and authentication timeouts. By default, where EAP is run over an
unreliable lower layer, the EAP retransmission timer SHOULD be
dynamically estimated. A maximum of 3-5 retransmissions is
suggested.
When run over a reliable lower layer (e.g., EAP over ISAKMP/TCP, as
within [PIC]), the authenticator retransmission timer SHOULD be set
to an infinite value, so that retransmissions do not occur at the EAP
layer. The peer may still maintain a timeout value so as to avoid
waiting indefinitely for a Request.
Where the authentication process requires user input, the measured
round trip times may be determined by user responsiveness rather than
network characteristics, so that dynamic RTO estimation may not be
helpful. Instead, the retransmission timer SHOULD be set so as to
provide sufficient time for the user to respond, with longer timeouts
required in certain cases, such as where Token Cards (see Section
5.6) are involved.
In order to provide the EAP authenticator with guidance as to the
appropriate timeout value, a hint can be communicated to the
authenticator by the backend authentication server (such as via the
RADIUS Session-Timeout attribute).
In order to dynamically estimate the EAP retransmission timer, the
algorithms for the estimation of SRTT, RTTVAR, and RTO described in
[RFC2988] are RECOMMENDED, including use of Karn's algorithm, with
the following potential modifications:
[a] In order to avoid synchronization behaviors that can occur with
fixed timers among distributed systems, the retransmission timer
is calculated with a jitter by using the RTO value and randomly
adding a value drawn between -RTOmin/2 and RTOmin/2. Alternative
calculations to create jitter MAY be used. These MUST be
pseudo-random. For a discussion of pseudo-random number
generation, see [RFC1750].
[b] When EAP is transported over a single link (as opposed to over
the Internet), smaller values of RTOinitial, RTOmin, and RTOmax
MAY be used. Recommended values are RTOinitial=1 second,
RTOmin=200ms, and RTOmax=20 seconds.
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[c] When EAP is transported over a single link (as opposed to over
the Internet), estimates MAY be done on a per-authenticator
basis, rather than a per-session basis. This enables the
retransmission estimate to make the most use of information on
link-layer behavior.
[d] An EAP implementation MAY clear SRTT and RTTVAR after backing off
the timer multiple times, as it is likely that the current SRTT
and RTTVAR are bogus in this situation. Once SRTT and RTTVAR are
cleared, they should be initialized with the next RTT sample
taken as described in [RFC2988] equation 2.2.
5. Initial EAP Request/Response Types
This section defines the initial set of EAP Types used in Request/
Response exchanges. More Types may be defined in future documents.
The Type field is one octet and identifies the structure of an EAP
Request or Response packet. The first 3 Types are considered special
case Types.
The remaining Types define authentication exchanges. Nak (Type 3) or
Expanded Nak (Type 254) are valid only for Response packets, they
MUST NOT be sent in a Request.
All EAP implementations MUST support Types 1-4, which are defined in
this document, and SHOULD support Type 254. Implementations MAY
support other Types defined here or in future RFCs.
1 Identity
2 Notification
3 Nak (Response only)
4 MD5-Challenge
5 One Time Password (OTP)
6 Generic Token Card (GTC)
254 Expanded Types
255 Experimental use
EAP methods MAY support authentication based on shared secrets. If
the shared secret is a passphrase entered by the user,
implementations MAY support entering passphrases with non-ASCII
characters. In this case, the input should be processed using an
appropriate stringprep [RFC3454] profile, and encoded in octets using
UTF-8 encoding [RFC2279]. A preliminary version of a possible
stringprep profile is described in [SASLPREP].
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5.1. Identity
Description
The Identity Type is used to query the identity of the peer.
Generally, the authenticator will issue this as the initial
Request. An optional displayable message MAY be included to
prompt the peer in the case where there is an expectation of
interaction with a user. A Response of Type 1 (Identity) SHOULD
be sent in Response to a Request with a Type of 1 (Identity).
Some EAP implementations piggy-back various options into the
Identity Request after a NUL-character. By default, an EAP
implementation SHOULD NOT assume that an Identity Request or
Response can be larger than 1020 octets.
It is RECOMMENDED that the Identity Response be used primarily for
routing purposes and selecting which EAP method to use. EAP
Methods SHOULD include a method-specific mechanism for obtaining
the identity, so that they do not have to rely on the Identity
Response. Identity Requests and Responses are sent in cleartext,
so an attacker may snoop on the identity, or even modify or spoof
identity exchanges. To address these threats, it is preferable
for an EAP method to include an identity exchange that supports
per-packet authentication, integrity and replay protection, and
confidentiality. The Identity Response may not be the appropriate
identity for the method; it may have been truncated or obfuscated
so as to provide privacy, or it may have been decorated for
routing purposes. Where the peer is configured to only accept
authentication methods supporting protected identity exchanges,
the peer MAY provide an abbreviated Identity Response (such as
omitting the peer-name portion of the NAI [RFC2486]). For further
discussion of identity protection, see Section 7.3.
Implementation Note: The peer MAY obtain the Identity via user input.
It is suggested that the authenticator retry the Identity Request in
the case of an invalid Identity or authentication failure to allow
for potential typos on the part of the user. It is suggested that
the Identity Request be retried a minimum of 3 times before
terminating the authentication. The Notification Request MAY be used
to indicate an invalid authentication attempt prior to transmitting a
new Identity Request (optionally, the failure MAY be indicated within
the message of the new Identity Request itself).
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Type
1
Type-Data
This field MAY contain a displayable message in the Request,
containing UTF-8 encoded ISO 10646 characters [RFC2279]. Where
the Request contains a null, only the portion of the field prior
to the null is displayed. If the Identity is unknown, the
Identity Response field should be zero bytes in length. The
Identity Response field MUST NOT be null terminated. In all
cases, the length of the Type-Data field is derived from the
Length field of the Request/Response packet.
Security Claims (see Section 7.2):
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: N/A
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.2. Notification
Description
The Notification Type is optionally used to convey a displayable
message from the authenticator to the peer. An authenticator MAY
send a Notification Request to the peer at any time when there is
no outstanding Request, prior to completion of an EAP
authentication method. The peer MUST respond to a Notification
Request with a Notification Response unless the EAP authentication
method specification prohibits the use of Notification messages.
In any case, a Nak Response MUST NOT be sent in response to a
Notification Request. Note that the default maximum length of a
Notification Request is 1020 octets. By default, this leaves at
most 1015 octets for the human readable message.
Aboba, et al. Standards Track [Page 29]
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An EAP method MAY indicate within its specification that
Notification messages must not be sent during that method. In
this case, the peer MUST silently discard Notification Requests
from the point where an initial Request for that Type is answered
with a Response of the same Type.
The peer SHOULD display this message to the user or log it if it
cannot be displayed. The Notification Type is intended to provide
an acknowledged notification of some imperative nature, but it is
not an error indication, and therefore does not change the state
of the peer. Examples include a password with an expiration time
that is about to expire, an OTP sequence integer which is nearing
0, an authentication failure warning, etc. In most circumstances,
Notification should not be required.
Type
2
Type-Data
The Type-Data field in the Request contains a displayable message
greater than zero octets in length, containing UTF-8 encoded ISO
10646 characters [RFC2279]. The length of the message is
determined by the Length field of the Request packet. The message
MUST NOT be null terminated. A Response MUST be sent in reply to
the Request with a Type field of 2 (Notification). The Type-Data
field of the Response is zero octets in length. The Response
should be sent immediately (independent of how the message is
displayed or logged).
Security Claims (see Section 7.2):
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: N/A
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
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5.3. Nak
5.3.1. Legacy Nak
Description
The legacy Nak Type is valid only in Response messages. It is
sent in reply to a Request where the desired authentication Type
is unacceptable. Authentication Types are numbered 4 and above.
The Response contains one or more authentication Types desired by
the Peer. Type zero (0) is used to indicate that the sender has
no viable alternatives, and therefore the authenticator SHOULD NOT
send another Request after receiving a Nak Response containing a
zero value.
Since the legacy Nak Type is valid only in Responses and has very
limited functionality, it MUST NOT be used as a general purpose
error indication, such as for communication of error messages, or
negotiation of parameters specific to a particular EAP method.
Code
2 for Response.
Identifier
The Identifier field is one octet and aids in matching Responses
with Requests. The Identifier field of a legacy Nak Response MUST
match the Identifier field of the Request packet that it is sent
in response to.
Length
>=6
Type
3
Type-Data
Where a peer receives a Request for an unacceptable authentication
Type (4-253,255), or a peer lacking support for Expanded Types
receives a Request for Type 254, a Nak Response (Type 3) MUST be
sent. The Type-Data field of the Nak Response (Type 3) MUST
contain one or more octets indicating the desired authentication
Type(s), one octet per Type, or the value zero (0) to indicate no
proposed alternative. A peer supporting Expanded Types that
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receives a Request for an unacceptable authentication Type (4-253,
255) MAY include the value 254 in the Nak Response (Type 3) to
indicate the desire for an Expanded authentication Type. If the
authenticator can accommodate this preference, it will respond
with an Expanded Type Request (Type 254).
Security Claims (see Section 7.2):
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: N/A
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.3.2. Expanded Nak
Description
The Expanded Nak Type is valid only in Response messages. It MUST
be sent only in reply to a Request of Type 254 (Expanded Type)
where the authentication Type is unacceptable. The Expanded Nak
Type uses the Expanded Type format itself, and the Response
contains one or more authentication Types desired by the peer, all
in Expanded Type format. Type zero (0) is used to indicate that
the sender has no viable alternatives. The general format of the
Expanded Type is described in Section 5.7.
Since the Expanded Nak Type is valid only in Responses and has
very limited functionality, it MUST NOT be used as a general
purpose error indication, such as for communication of error
messages, or negotiation of parameters specific to a particular
EAP method.
Code
2 for Response.
Aboba, et al. Standards Track [Page 32]
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Identifier
The Identifier field is one octet and aids in matching Responses
with Requests. The Identifier field of an Expanded Nak Response
MUST match the Identifier field of the Request packet that it is
sent in response to.
Length
>=20
Type
254
Vendor-Id
0 (IETF)
Vendor-Type
3 (Nak)
Vendor-Data
The Expanded Nak Type is only sent when the Request contains an
Expanded Type (254) as defined in Section 5.7. The Vendor-Data
field of the Nak Response MUST contain one or more authentication
Types (4 or greater), all in expanded format, 8 octets per Type,
or the value zero (0), also in Expanded Type format, to indicate
no proposed alternative. The desired authentication Types may
include a mixture of Vendor-Specific and IETF Types. For example,
an Expanded Nak Response indicating a preference for OTP (Type 5),
and an MIT (Vendor-Id=20) Expanded Type of 6 would appear as
follows:
Aboba, et al. Standards Track [Page 33]
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Identifier | Length=28 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 (Nak) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 (OTP) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 20 (MIT) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Expanded Nak Response indicating a no desired alternative would
appear as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Identifier | Length=20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 (Nak) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 (No alternative) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Security Claims (see Section 7.2):
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: N/A
Fast reconnect: No
Crypt. binding: N/A
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Session independence: N/A
Fragmentation: No
Channel binding: No
5.4. MD5-Challenge
Description
The MD5-Challenge Type is analogous to the PPP CHAP protocol
[RFC1994] (with MD5 as the specified algorithm). The Request
contains a "challenge" message to the peer. A Response MUST be
sent in reply to the Request. The Response MAY be either of Type
4 (MD5-Challenge), Nak (Type 3), or Expanded Nak (Type 254). The
Nak reply indicates the peer's desired authentication Type(s).
EAP peer and EAP server implementations MUST support the MD5-
Challenge mechanism. An authenticator that supports only pass-
through MUST allow communication with a backend authentication
server that is capable of supporting MD5-Challenge, although the
EAP authenticator implementation need not support MD5-Challenge
itself. However, if the EAP authenticator can be configured to
authenticate peers locally (e.g., not operate in pass-through),
then the requirement for support of the MD5-Challenge mechanism
applies.
Note that the use of the Identifier field in the MD5-Challenge
Type is different from that described in [RFC1994]. EAP allows
for retransmission of MD5-Challenge Request packets, while
[RFC1994] states that both the Identifier and Challenge fields
MUST change each time a Challenge (the CHAP equivalent of the
MD5-Challenge Request packet) is sent.
Note: [RFC1994] treats the shared secret as an octet string, and
does not specify how it is entered into the system (or if it is
handled by the user at all). EAP MD5-Challenge implementations
MAY support entering passphrases with non-ASCII characters. See
Section 5 for instructions how the input should be processed and
encoded into octets.
Type
4
Type-Data
The contents of the Type-Data field is summarized below. For
reference on the use of these fields, see the PPP Challenge
Handshake Authentication Protocol [RFC1994].
Aboba, et al. Standards Track [Page 35]
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value-Size | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Security Claims (see Section 7.2):
Auth. mechanism: Password or pre-shared key.
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: No
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.5. One-Time Password (OTP)
Description
The One-Time Password system is defined in "A One-Time Password
System" [RFC2289] and "OTP Extended Responses" [RFC2243]. The
Request contains an OTP challenge in the format described in
[RFC2289]. A Response MUST be sent in reply to the Request. The
Response MUST be of Type 5 (OTP), Nak (Type 3), or Expanded Nak
(Type 254). The Nak Response indicates the peer's desired
authentication Type(s). The EAP OTP method is intended for use
with the One-Time Password system only, and MUST NOT be used to
provide support for cleartext passwords.
Type
5
Aboba, et al. Standards Track [Page 36]
RFC 3748 EAP June 2004
Type-Data
The Type-Data field contains the OTP "challenge" as a displayable
message in the Request. In the Response, this field is used for
the 6 words from the OTP dictionary [RFC2289]. The messages MUST
NOT be null terminated. The length of the field is derived from
the Length field of the Request/Reply packet.
Note: [RFC2289] does not specify how the secret pass-phrase is
entered by the user, or how the pass-phrase is converted into
octets. EAP OTP implementations MAY support entering passphrases
with non-ASCII characters. See Section 5 for instructions on how
the input should be processed and encoded into octets.
Security Claims (see Section 7.2):
Auth. mechanism: One-Time Password
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: Yes
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: No
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.6. Generic Token Card (GTC)
Description
The Generic Token Card Type is defined for use with various Token
Card implementations which require user input. The Request
contains a displayable message and the Response contains the Token
Card information necessary for authentication. Typically, this
would be information read by a user from the Token card device and
entered as ASCII text. A Response MUST be sent in reply to the
Request. The Response MUST be of Type 6 (GTC), Nak (Type 3), or
Expanded Nak (Type 254). The Nak Response indicates the peer's
desired authentication Type(s). The EAP GTC method is intended
for use with the Token Cards supporting challenge/response
Aboba, et al. Standards Track [Page 37]
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authentication and MUST NOT be used to provide support for
cleartext passwords in the absence of a protected tunnel with
server authentication.
Type
6
Type-Data
The Type-Data field in the Request contains a displayable message
greater than zero octets in length. The length of the message is
determined by the Length field of the Request packet. The message
MUST NOT be null terminated. A Response MUST be sent in reply to
the Request with a Type field of 6 (Generic Token Card). The
Response contains data from the Token Card required for
authentication. The length of the data is determined by the
Length field of the Response packet.
EAP GTC implementations MAY support entering a response with non-
ASCII characters. See Section 5 for instructions how the input
should be processed and encoded into octets.
Security Claims (see Section 7.2):
Auth. mechanism: Hardware token.
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: No
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.7. Expanded Types
Description
Since many of the existing uses of EAP are vendor-specific, the
Expanded method Type is available to allow vendors to support
their own Expanded Types not suitable for general usage.
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RFC 3748 EAP June 2004
The Expanded Type is also used to expand the global Method Type
space beyond the original 255 values. A Vendor-Id of 0 maps the
original 255 possible Types onto a space of 2^32-1 possible Types.
(Type 0 is only used in a Nak Response to indicate no acceptable
alternative).
An implementation that supports the Expanded attribute MUST treat
EAP Types that are less than 256 equivalently, whether they appear
as a single octet or as the 32-bit Vendor-Type within an Expanded
Type where Vendor-Id is 0. Peers not equipped to interpret the
Expanded Type MUST send a Nak as described in Section 5.3.1, and
negotiate a more suitable authentication method.
A summary of the Expanded Type format is shown below. The fields
are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
254 for Expanded Type
Vendor-Id
The Vendor-Id is 3 octets and represents the SMI Network
Management Private Enterprise Code of the Vendor in network byte
order, as allocated by IANA. A Vendor-Id of zero is reserved for
use by the IETF in providing an expanded global EAP Type space.
Vendor-Type
The Vendor-Type field is four octets and represents the vendor-
specific method Type.
If the Vendor-Id is zero, the Vendor-Type field is an extension
and superset of the existing namespace for EAP Types. The first
256 Types are reserved for compatibility with single-octet EAP
Types that have already been assigned or may be assigned in the
future. Thus, EAP Types from 0 through 255 are semantically
identical, whether they appear as single octet EAP Types or as
Aboba, et al. Standards Track [Page 39]
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Vendor-Types when Vendor-Id is zero. There is one exception to
this rule: Expanded Nak and Legacy Nak packets share the same
Type, but must be treated differently because they have a
different format.
Vendor-Data
The Vendor-Data field is defined by the vendor. Where a Vendor-Id
of zero is present, the Vendor-Data field will be used for
transporting the contents of EAP methods of Types defined by the
IETF.
5.8. Experimental
Description
The Experimental Type has no fixed format or content. It is
intended for use when experimenting with new EAP Types. This Type
is intended for experimental and testing purposes. No guarantee
is made for interoperability between peers using this Type, as
outlined in [RFC3692].
Type
255
Type-Data
Undefined
6. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the EAP
protocol, in accordance with BCP 26, [RFC2434].
There are two name spaces in EAP that require registration: Packet
Codes and method Types.
EAP is not intended as a general-purpose protocol, and allocations
SHOULD NOT be made for purposes unrelated to authentication.
The following terms are used here with the meanings defined in BCP
26: "name space", "assigned value", "registration".
The following policies are used here with the meanings defined in BCP
26: "Private Use", "First Come First Served", "Expert Review",
"Specification Required", "IETF Consensus", "Standards Action".
Aboba, et al. Standards Track [Page 40]
RFC 3748 EAP June 2004
For registration requests where a Designated Expert should be
consulted, the responsible IESG area director should appoint the
Designated Expert. The intention is that any allocation will be
accompanied by a published RFC. But in order to allow for the
allocation of values prior to the RFC being approved for publication,
the Designated Expert can approve allocations once it seems clear
that an RFC will be published. The Designated expert will post a
request to the EAP WG mailing list (or a successor designated by the
Area Director) for comment and review, including an Internet-Draft.
Before a period of 30 days has passed, the Designated Expert will
either approve or deny the registration request and publish a notice
of the decision to the EAP WG mailing list or its successor, as well
as informing IANA. A denial notice must be justified by an
explanation, and in the cases where it is possible, concrete
suggestions on how the request can be modified so as to become
acceptable should be provided.
6.1. Packet Codes
Packet Codes have a range from 1 to 255, of which 1-4 have been
allocated. Because a new Packet Code has considerable impact on
interoperability, a new Packet Code requires Standards Action, and
should be allocated starting at 5.
6.2. Method Types
The original EAP method Type space has a range from 1 to 255, and is
the scarcest resource in EAP, and thus must be allocated with care.
Method Types 1-45 have been allocated, with 20 available for re-use.
Method Types 20 and 46-191 may be allocated on the advice of a
Designated Expert, with Specification Required.
Allocation of blocks of method Types (more than one for a given
purpose) should require IETF Consensus. EAP Type Values 192-253 are
reserved and allocation requires Standards Action.
Method Type 254 is allocated for the Expanded Type. Where the
Vendor-Id field is non-zero, the Expanded Type is used for functions
specific only to one vendor's implementation of EAP, where no
interoperability is deemed useful. When used with a Vendor-Id of
zero, method Type 254 can also be used to provide for an expanded
IETF method Type space. Method Type values 256-4294967295 may be
allocated after Type values 1-191 have been allocated, on the advice
of a Designated Expert, with Specification Required.
Method Type 255 is allocated for Experimental use, such as testing of
new EAP methods before a permanent Type is allocated.
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7. Security Considerations
This section defines a generic threat model as well as the EAP method
security claims mitigating those threats.
It is expected that the generic threat model and corresponding
security claims will used to define EAP method requirements for use
in specific environments. An example of such a requirements analysis
is provided in [IEEE-802.11i-req]. A security claims section is
required in EAP method specifications, so that EAP methods can be
evaluated against the requirements.
7.1. Threat Model
EAP was developed for use with PPP [RFC1661] and was later adapted
for use in wired IEEE 802 networks [IEEE-802] in [IEEE-802.1X].
Subsequently, EAP has been proposed for use on wireless LAN networks
and over the Internet. In all these situations, it is possible for
an attacker to gain access to links over which EAP packets are
transmitted. For example, attacks on telephone infrastructure are
documented in [DECEPTION].
An attacker with access to the link may carry out a number of
attacks, including:
[1] An attacker may try to discover user identities by snooping
authentication traffic.
[2] An attacker may try to modify or spoof EAP packets.
[3] An attacker may launch denial of service attacks by spoofing
lower layer indications or Success/Failure packets, by replaying
EAP packets, or by generating packets with overlapping
Identifiers.
[4] An attacker may attempt to recover the pass-phrase by mounting
an offline dictionary attack.
[5] An attacker may attempt to convince the peer to connect to an
untrusted network by mounting a man-in-the-middle attack.
[6] An attacker may attempt to disrupt the EAP negotiation in order
cause a weak authentication method to be selected.
[7] An attacker may attempt to recover keys by taking advantage of
weak key derivation techniques used within EAP methods.
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RFC 3748 EAP June 2004
[8] An attacker may attempt to take advantage of weak ciphersuites
subsequently used after the EAP conversation is complete.
[9] An attacker may attempt to perform downgrading attacks on lower
layer ciphersuite negotiation in order to ensure that a weaker
ciphersuite is used subsequently to EAP authentication.
[10] An attacker acting as an authenticator may provide incorrect
information to the EAP peer and/or server via out-of-band
mechanisms (such as via a AAA or lower layer protocol). This
includes impersonating another authenticator, or providing
inconsistent information to the peer and EAP server.
Depending on the lower layer, these attacks may be carried out
without requiring physical proximity. Where EAP is used over
wireless networks, EAP packets may be forwarded by authenticators
(e.g., pre-authentication) so that the attacker need not be within
the coverage area of an authenticator in order to carry out an attack
on it or its peers. Where EAP is used over the Internet, attacks may
be carried out at an even greater distance.
7.2. Security Claims
In order to clearly articulate the security provided by an EAP
method, EAP method specifications MUST include a Security Claims
section, including the following declarations:
[a] Mechanism. This is a statement of the authentication technology:
certificates, pre-shared keys, passwords, token cards, etc.
[b] Security claims. This is a statement of the claimed security
properties of the method, using terms defined in Section 7.2.1:
mutual authentication, integrity protection, replay protection,
confidentiality, key derivation, dictionary attack resistance,
fast reconnect, cryptographic binding. The Security Claims
section of an EAP method specification SHOULD provide
justification for the claims that are made. This can be
accomplished by including a proof in an Appendix, or including a
reference to a proof.
[c] Key strength. If the method derives keys, then the effective key
strength MUST be estimated. This estimate is meant for potential
users of the method to determine if the keys produced are strong
enough for the intended application.
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RFC 3748 EAP June 2004
The effective key strength SHOULD be stated as a number of bits,
defined as follows: If the effective key strength is N bits, the
best currently known methods to recover the key (with non-
negligible probability) require, on average, an effort comparable
to 2^(N-1) operations of a typical block cipher. The statement
SHOULD be accompanied by a short rationale, explaining how this
number was derived. This explanation SHOULD include the
parameters required to achieve the stated key strength based on
current knowledge of the algorithms.
(Note: Although it is difficult to define what "comparable
effort" and "typical block cipher" exactly mean, reasonable
approximations are sufficient here. Refer to e.g. [SILVERMAN]
for more discussion.)
The key strength depends on the methods used to derive the keys.
For instance, if keys are derived from a shared secret (such as a
password or a long-term secret), and possibly some public
information such as nonces, the effective key strength is limited
by the strength of the long-term secret (assuming that the
derivation procedure is computationally simple). To take another
example, when using public key algorithms, the strength of the
symmetric key depends on the strength of the public keys used.
[d] Description of key hierarchy. EAP methods deriving keys MUST
either provide a reference to a key hierarchy specification, or
describe how Master Session Keys (MSKs) and Extended Master
Session Keys (EMSKs) are to be derived.
[e] Indication of vulnerabilities. In addition to the security
claims that are made, the specification MUST indicate which of
the security claims detailed in Section 7.2.1 are NOT being made.
7.2.1. Security Claims Terminology for EAP Methods
These terms are used to describe the security properties of EAP
methods:
Protected ciphersuite negotiation
This refers to the ability of an EAP method to negotiate the
ciphersuite used to protect the EAP conversation, as well as to
integrity protect the negotiation. It does not refer to the
ability to negotiate the ciphersuite used to protect data.
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Mutual authentication
This refers to an EAP method in which, within an interlocked
exchange, the authenticator authenticates the peer and the peer
authenticates the authenticator. Two independent one-way methods,
running in opposite directions do not provide mutual
authentication as defined here.
Integrity protection
This refers to providing data origin authentication and protection
against unauthorized modification of information for EAP packets
(including EAP Requests and Responses). When making this claim, a
method specification MUST describe the EAP packets and fields
within the EAP packet that are protected.
Replay protection
This refers to protection against replay of an EAP method or its
messages, including success and failure result indications.
Confidentiality
This refers to encryption of EAP messages, including EAP Requests
and Responses, and success and failure result indications. A
method making this claim MUST support identity protection (see
Section 7.3).
Key derivation
This refers to the ability of the EAP method to derive exportable
keying material, such as the Master Session Key (MSK), and
Extended Master Session Key (EMSK). The MSK is used only for
further key derivation, not directly for protection of the EAP
conversation or subsequent data. Use of the EMSK is reserved.
Key strength
If the effective key strength is N bits, the best currently known
methods to recover the key (with non-negligible probability)
require, on average, an effort comparable to 2^(N-1) operations of
a typical block cipher.
Dictionary attack resistance
Where password authentication is used, passwords are commonly
selected from a small set (as compared to a set of N-bit keys),
which raises a concern about dictionary attacks. A method may be
said to provide protection against dictionary attacks if, when it
uses a password as a secret, the method does not allow an offline
attack that has a work factor based on the number of passwords in
an attacker's dictionary.
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Fast reconnect
The ability, in the case where a security association has been
previously established, to create a new or refreshed security
association more efficiently or in a smaller number of round-
trips.
Cryptographic binding
The demonstration of the EAP peer to the EAP server that a single
entity has acted as the EAP peer for all methods executed within a
tunnel method. Binding MAY also imply that the EAP server
demonstrates to the peer that a single entity has acted as the EAP
server for all methods executed within a tunnel method. If
executed correctly, binding serves to mitigate man-in-the-middle
vulnerabilities.
Session independence
The demonstration that passive attacks (such as capture of the EAP
conversation) or active attacks (including compromise of the MSK
or EMSK) does not enable compromise of subsequent or prior MSKs or
EMSKs.
Fragmentation
This refers to whether an EAP method supports fragmentation and
reassembly. As noted in Section 3.1, EAP methods should support
fragmentation and reassembly if EAP packets can exceed the minimum
MTU of 1020 octets.
Channel binding
The communication within an EAP method of integrity-protected
channel properties such as endpoint identifiers which can be
compared to values communicated via out of band mechanisms (such
as via a AAA or lower layer protocol).
Note: This list of security claims is not exhaustive. Additional
properties, such as additional denial-of-service protection, may be
relevant as well.
7.3. Identity Protection
An Identity exchange is optional within the EAP conversation.
Therefore, it is possible to omit the Identity exchange entirely, or
to use a method-specific identity exchange once a protected channel
has been established.
However, where roaming is supported as described in [RFC2607], it may
be necessary to locate the appropriate backend authentication server
before the authentication conversation can proceed. The realm
portion of the Network Access Identifier (NAI) [RFC2486] is typically
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included within the EAP-Response/Identity in order to enable the
authentication exchange to be routed to the appropriate backend
authentication server. Therefore, while the peer-name portion of the
NAI may be omitted in the EAP-Response/Identity where proxies or
relays are present, the realm portion may be required.
It is possible for the identity in the identity response to be
different from the identity authenticated by the EAP method. This
may be intentional in the case of identity privacy. An EAP method
SHOULD use the authenticated identity when making access control
decisions.
7.4. Man-in-the-Middle Attacks
Where EAP is tunneled within another protocol that omits peer
authentication, there exists a potential vulnerability to a man-in-
the-middle attack. For details, see [BINDING] and [MITM].
As noted in Section 2.1, EAP does not permit untunneled sequences of
authentication methods. Were a sequence of EAP authentication
methods to be permitted, the peer might not have proof that a single
entity has acted as the authenticator for all EAP methods within the
sequence. For example, an authenticator might terminate one EAP
method, then forward the next method in the sequence to another party
without the peer's knowledge or consent. Similarly, the
authenticator might not have proof that a single entity has acted as
the peer for all EAP methods within the sequence.
Tunneling EAP within another protocol enables an attack by a rogue
EAP authenticator tunneling EAP to a legitimate server. Where the
tunneling protocol is used for key establishment but does not require
peer authentication, an attacker convincing a legitimate peer to
connect to it will be able to tunnel EAP packets to a legitimate
server, successfully authenticating and obtaining the key. This
allows the attacker to successfully establish itself as a man-in-
the-middle, gaining access to the network, as well as the ability to
decrypt data traffic between the legitimate peer and server.
This attack may be mitigated by the following measures:
[a] Requiring mutual authentication within EAP tunneling mechanisms.
[b] Requiring cryptographic binding between the EAP tunneling
protocol and the tunneled EAP methods. Where cryptographic
binding is supported, a mechanism is also needed to protect
against downgrade attacks that would bypass it. For further
details on cryptographic binding, see [BINDING].
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[c] Limiting the EAP methods authorized for use without protection,
based on peer and authenticator policy.
[d] Avoiding the use of tunnels when a single, strong method is
available.
7.5. Packet Modification Attacks
While EAP methods may support per-packet data origin authentication,
integrity, and replay protection, support is not provided within the
EAP layer.
Since the Identifier is only a single octet, it is easy to guess,
allowing an attacker to successfully inject or replay EAP packets.
An attacker may also modify EAP headers (Code, Identifier, Length,
Type) within EAP packets where the header is unprotected. This could
cause packets to be inappropriately discarded or misinterpreted.
To protect EAP packets against modification, spoofing, or replay,
methods supporting protected ciphersuite negotiation, mutual
authentication, and key derivation, as well as integrity and replay
protection, are recommended. See Section 7.2.1 for definitions of
these security claims.
Method-specific MICs may be used to provide protection. If a per-
packet MIC is employed within an EAP method, then peers,
authentication servers, and authenticators not operating in pass-
through mode MUST validate the MIC. MIC validation failures SHOULD
be logged. Whether a MIC validation failure is considered a fatal
error or not is determined by the EAP method specification.
It is RECOMMENDED that methods providing integrity protection of EAP
packets include coverage of all the EAP header fields, including the
Code, Identifier, Length, Type, and Type-Data fields.
Since EAP messages of Types Identity, Notification, and Nak do not
include their own MIC, it may be desirable for the EAP method MIC to
cover information contained within these messages, as well as the
header of each EAP message.
To provide protection, EAP also may be encapsulated within a
protected channel created by protocols such as ISAKMP [RFC2408], as
is done in [IKEv2] or within TLS [RFC2246]. However, as noted in
Section 7.4, EAP tunneling may result in a man-in-the-middle
vulnerability.
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Existing EAP methods define message integrity checks (MICs) that
cover more than one EAP packet. For example, EAP-TLS [RFC2716]
defines a MIC over a TLS record that could be split into multiple
fragments; within the FINISHED message, the MIC is computed over
previous messages. Where the MIC covers more than one EAP packet, a
MIC validation failure is typically considered a fatal error.
Within EAP-TLS [RFC2716], a MIC validation failure is treated as a
fatal error, since that is what is specified in TLS [RFC2246].
However, it is also possible to develop EAP methods that support
per-packet MICs, and respond to verification failures by silently
discarding the offending packet.
In this document, descriptions of EAP message handling assume that
per-packet MIC validation, where it occurs, is effectively performed
as though it occurs before sending any responses or changing the
state of the host which received the packet.
7.6. Dictionary Attacks
Password authentication algorithms such as EAP-MD5, MS-CHAPv1
[RFC2433], and Kerberos V [RFC1510] are known to be vulnerable to
dictionary attacks. MS-CHAPv1 vulnerabilities are documented in
[PPTPv1]; MS-CHAPv2 vulnerabilities are documented in [PPTPv2];
Kerberos vulnerabilities are described in [KRBATTACK], [KRBLIM], and
[KERB4WEAK].
In order to protect against dictionary attacks, authentication
methods resistant to dictionary attacks (as defined in Section 7.2.1)
are recommended.
If an authentication algorithm is used that is known to be vulnerable
to dictionary attacks, then the conversation may be tunneled within a
protected channel in order to provide additional protection.
However, as noted in Section 7.4, EAP tunneling may result in a man-
in-the-middle vulnerability, and therefore dictionary attack
resistant methods are preferred.
7.7. Connection to an Untrusted Network
With EAP methods supporting one-way authentication, such as EAP-MD5,
the peer does not authenticate the authenticator, making the peer
vulnerable to attack by a rogue authenticator. Methods supporting
mutual authentication (as defined in Section 7.2.1) address this
vulnerability.
In EAP there is no requirement that authentication be full duplex or
that the same protocol be used in both directions. It is perfectly
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acceptable for different protocols to be used in each direction.
This will, of course, depend on the specific protocols negotiated.
However, in general, completing a single unitary mutual
authentication is preferable to two one-way authentications, one in
each direction. This is because separate authentications that are
not bound cryptographically so as to demonstrate they are part of the
same session are subject to man-in-the-middle attacks, as discussed
in Section 7.4.
7.8. Negotiation Attacks
In a negotiation attack, the attacker attempts to convince the peer
and authenticator to negotiate a less secure EAP method. EAP does
not provide protection for Nak Response packets, although it is
possible for a method to include coverage of Nak Responses within a
method-specific MIC.
Within or associated with each authenticator, it is not anticipated
that a particular named peer will support a choice of methods. This
would make the peer vulnerable to attacks that negotiate the least
secure method from among a set. Instead, for each named peer, there
SHOULD be an indication of exactly one method used to authenticate
that peer name. If a peer needs to make use of different
authentication methods under different circumstances, then distinct
identities SHOULD be employed, each of which identifies exactly one
authentication method.
7.9. Implementation Idiosyncrasies
The interaction of EAP with lower layers such as PPP and IEEE 802 are
highly implementation dependent.
For example, upon failure of authentication, some PPP implementations
do not terminate the link, instead limiting traffic in Network-Layer
Protocols to a filtered subset, which in turn allows the peer the
opportunity to update secrets or send mail to the network
administrator indicating a problem. Similarly, while an
authentication failure will result in denied access to the controlled
port in [IEEE-802.1X], limited traffic may be permitted on the
uncontrolled port.
In EAP there is no provision for retries of failed authentication.
However, in PPP the LCP state machine can renegotiate the
authentication protocol at any time, thus allowing a new attempt.
Similarly, in IEEE 802.1X the Supplicant or Authenticator can re-
authenticate at any time. It is recommended that any counters used
for authentication failure not be reset until after successful
authentication, or subsequent termination of the failed link.
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7.10. Key Derivation
It is possible for the peer and EAP server to mutually authenticate
and derive keys. In order to provide keying material for use in a
subsequently negotiated ciphersuite, an EAP method supporting key
derivation MUST export a Master Session Key (MSK) of at least 64
octets, and an Extended Master Session Key (EMSK) of at least 64
octets. EAP Methods deriving keys MUST provide for mutual
authentication between the EAP peer and the EAP Server.
The MSK and EMSK MUST NOT be used directly to protect data; however,
they are of sufficient size to enable derivation of a AAA-Key
subsequently used to derive Transient Session Keys (TSKs) for use
with the selected ciphersuite. Each ciphersuite is responsible for
specifying how to derive the TSKs from the AAA-Key.
The AAA-Key is derived from the keying material exported by the EAP
method (MSK and EMSK). This derivation occurs on the AAA server. In
many existing protocols that use EAP, the AAA-Key and MSK are
equivalent, but more complicated mechanisms are possible (see
[KEYFRAME] for details).
EAP methods SHOULD ensure the freshness of the MSK and EMSK, even in
cases where one party may not have a high quality random number
generator. A RECOMMENDED method is for each party to provide a nonce
of at least 128 bits, used in the derivation of the MSK and EMSK.
EAP methods export the MSK and EMSK, but not Transient Session Keys
so as to allow EAP methods to be ciphersuite and media independent.
Keying material exported by EAP methods MUST be independent of the
ciphersuite negotiated to protect data.
Depending on the lower layer, EAP methods may run before or after
ciphersuite negotiation, so that the selected ciphersuite may not be
known to the EAP method. By providing keying material usable with
any ciphersuite, EAP methods can used with a wide range of
ciphersuites and media.
In order to preserve algorithm independence, EAP methods deriving
keys SHOULD support (and document) the protected negotiation of the
ciphersuite used to protect the EAP conversation between the peer and
server. This is distinct from the ciphersuite negotiated between the
peer and authenticator, used to protect data.
The strength of Transient Session Keys (TSKs) used to protect data is
ultimately dependent on the strength of keys generated by the EAP
method. If an EAP method cannot produce keying material of
sufficient strength, then the TSKs may be subject to a brute force
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attack. In order to enable deployments requiring strong keys, EAP
methods supporting key derivation SHOULD be capable of generating an
MSK and EMSK, each with an effective key strength of at least 128
bits.
Methods supporting key derivation MUST demonstrate cryptographic
separation between the MSK and EMSK branches of the EAP key
hierarchy. Without violating a fundamental cryptographic assumption
(such as the non-invertibility of a one-way function), an attacker
recovering the MSK or EMSK MUST NOT be able to recover the other
quantity with a level of effort less than brute force.
Non-overlapping substrings of the MSK MUST be cryptographically
separate from each other, as defined in Section 7.2.1. That is,
knowledge of one substring MUST NOT help in recovering some other
substring without breaking some hard cryptographic assumption. This
is required because some existing ciphersuites form TSKs by simply
splitting the AAA-Key to pieces of appropriate length. Likewise,
non-overlapping substrings of the EMSK MUST be cryptographically
separate from each other, and from substrings of the MSK.
The EMSK is reserved for future use and MUST remain on the EAP peer
and EAP server where it is derived; it MUST NOT be transported to, or
shared with, additional parties, or used to derive any other keys.
(This restriction will be relaxed in a future document that specifies
how the EMSK can be used.)
Since EAP does not provide for explicit key lifetime negotiation, EAP
peers, authenticators, and authentication servers MUST be prepared
for situations in which one of the parties discards the key state,
which remains valid on another party.
This specification does not provide detailed guidance on how EAP
methods derive the MSK and EMSK, how the AAA-Key is derived from the
MSK and/or EMSK, or how the TSKs are derived from the AAA-Key.
The development and validation of key derivation algorithms is
difficult, and as a result, EAP methods SHOULD re-use well
established and analyzed mechanisms for key derivation (such as those
specified in IKE [RFC2409] or TLS [RFC2246]), rather than inventing
new ones. EAP methods SHOULD also utilize well established and
analyzed mechanisms for MSK and EMSK derivation. Further details on
EAP Key Derivation are provided within [KEYFRAME].
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7.11. Weak Ciphersuites
If after the initial EAP authentication, data packets are sent
without per-packet authentication, integrity, and replay protection,
an attacker with access to the media can inject packets, "flip bits"
within existing packets, replay packets, or even hijack the session
completely. Without per-packet confidentiality, it is possible to
snoop data packets.
To protect against data modification, spoofing, or snooping, it is
recommended that EAP methods supporting mutual authentication and key
derivation (as defined by Section 7.2.1) be used, along with lower
layers providing per-packet confidentiality, authentication,
integrity, and replay protection.
Additionally, if the lower layer performs ciphersuite negotiation, it
should be understood that EAP does not provide by itself integrity
protection of that negotiation. Therefore, in order to avoid
downgrading attacks which would lead to weaker ciphersuites being
used, clients implementing lower layer ciphersuite negotiation SHOULD
protect against negotiation downgrading.
This can be done by enabling users to configure which ciphersuites
are acceptable as a matter of security policy, or the ciphersuite
negotiation MAY be authenticated using keying material derived from
the EAP authentication and a MIC algorithm agreed upon in advance by
lower-layer peers.
7.12. Link Layer
There are reliability and security issues with link layer indications
in PPP, IEEE 802 LANs, and IEEE 802.11 wireless LANs:
[a] PPP. In PPP, link layer indications such as LCP-Terminate (a
link failure indication) and NCP (a link success indication) are
not authenticated or integrity protected. They can therefore be
spoofed by an attacker with access to the link.
[b] IEEE 802. IEEE 802.1X EAPOL-Start and EAPOL-Logoff frames are
not authenticated or integrity protected. They can therefore be
spoofed by an attacker with access to the link.
[c] IEEE 802.11. In IEEE 802.11, link layer indications include
Disassociate and Deauthenticate frames (link failure
indications), and the first message of the 4-way handshake (link
success indication). These messages are not authenticated or
integrity protected, and although they are not forwardable, they
are spoofable by an attacker within range.
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In IEEE 802.11, IEEE 802.1X data frames may be sent as Class 3
unicast data frames, and are therefore forwardable. This implies
that while EAPOL-Start and EAPOL-Logoff messages may be authenticated
and integrity protected, they can be spoofed by an authenticated
attacker far from the target when "pre-authentication" is enabled.
In IEEE 802.11, a "link down" indication is an unreliable indication
of link failure, since wireless signal strength can come and go and
may be influenced by radio frequency interference generated by an
attacker. To avoid unnecessary resets, it is advisable to damp these
indications, rather than passing them directly to the EAP. Since EAP
supports retransmission, it is robust against transient connectivity
losses.
7.13. Separation of Authenticator and Backend Authentication Server
It is possible for the EAP peer and EAP server to mutually
authenticate and derive a AAA-Key for a ciphersuite used to protect
subsequent data traffic. This does not present an issue on the peer,
since the peer and EAP client reside on the same machine; all that is
required is for the client to derive the AAA-Key from the MSK and
EMSK exported by the EAP method, and to subsequently pass a Transient
Session Key (TSK) to the ciphersuite module.
However, in the case where the authenticator and authentication
server reside on different machines, there are several implications
for security.
[a] Authentication will occur between the peer and the authentication
server, not between the peer and the authenticator. This means
that it is not possible for the peer to validate the identity of
the authenticator that it is speaking to, using EAP alone.
[b] As discussed in [RFC3579], the authenticator is dependent on the
AAA protocol in order to know the outcome of an authentication
conversation, and does not look at the encapsulated EAP packet
(if one is present) to determine the outcome. In practice, this
implies that the AAA protocol spoken between the authenticator
and authentication server MUST support per-packet authentication,
integrity, and replay protection.
[c] After completion of the EAP conversation, where lower layer
security services such as per-packet confidentiality,
authentication, integrity, and replay protection will be enabled,
a secure association protocol SHOULD be run between the peer and
authenticator in order to provide mutual authentication between
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the peer and authenticator, guarantee liveness of transient
session keys, provide protected ciphersuite and capabilities
negotiation for subsequent data, and synchronize key usage.
[d] A AAA-Key derived from the MSK and/or EMSK negotiated between the
peer and authentication server MAY be transmitted to the
authenticator. Therefore, a mechanism needs to be provided to
transmit the AAA-Key from the authentication server to the
authenticator that needs it. The specification of the AAA-key
derivation, transport, and wrapping mechanisms is outside the
scope of this document. Further details on AAA-Key Derivation
are provided within [KEYFRAME].
7.14. Cleartext Passwords
This specification does not define a mechanism for cleartext password
authentication. The omission is intentional. Use of cleartext
passwords would allow the password to be captured by an attacker with
access to a link over which EAP packets are transmitted.
Since protocols encapsulating EAP, such as RADIUS [RFC3579], may not
provide confidentiality, EAP packets may be subsequently encapsulated
for transport over the Internet where they may be captured by an
attacker.
As a result, cleartext passwords cannot be securely used within EAP,
except where encapsulated within a protected tunnel with server
authentication. Some of the same risks apply to EAP methods without
dictionary attack resistance, as defined in Section 7.2.1. For
details, see Section 7.6.
7.15. Channel Binding
It is possible for a compromised or poorly implemented EAP
authenticator to communicate incorrect information to the EAP peer
and/or server. This may enable an authenticator to impersonate
another authenticator or communicate incorrect information via out-
of-band mechanisms (such as via a AAA or lower layer protocol).
Where EAP is used in pass-through mode, the EAP peer typically does
not verify the identity of the pass-through authenticator, it only
verifies that the pass-through authenticator is trusted by the EAP
server. This creates a potential security vulnerability.
Section 4.3.7 of [RFC3579] describes how an EAP pass-through
authenticator acting as a AAA client can be detected if it attempts
to impersonate another authenticator (such by sending incorrect NAS-
Identifier [RFC2865], NAS-IP-Address [RFC2865] or NAS-IPv6-Address
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[RFC3162] attributes via the AAA protocol). However, it is possible
for a pass-through authenticator acting as a AAA client to provide
correct information to the AAA server while communicating misleading
information to the EAP peer via a lower layer protocol.
For example, it is possible for a compromised authenticator to
utilize another authenticator's Called-Station-Id or NAS-Identifier
in communicating with the EAP peer via a lower layer protocol, or for
a pass-through authenticator acting as a AAA client to provide an
incorrect peer Calling-Station-Id [RFC2865][RFC3580] to the AAA
server via the AAA protocol.
In order to address this vulnerability, EAP methods may support a
protected exchange of channel properties such as endpoint
identifiers, including (but not limited to): Called-Station-Id
[RFC2865][RFC3580], Calling-Station-Id [RFC2865][RFC3580], NAS-
Identifier [RFC2865], NAS-IP-Address [RFC2865], and NAS-IPv6-Address
[RFC3162].
Using such a protected exchange, it is possible to match the channel
properties provided by the authenticator via out-of-band mechanisms
against those exchanged within the EAP method. Where discrepancies
are found, these SHOULD be logged; additional actions MAY also be
taken, such as denying access.
7.16. Protected Result Indications
Within EAP, Success and Failure packets are neither acknowledged nor
integrity protected. Result indications improve resilience to loss
of Success and Failure packets when EAP is run over lower layers
which do not support retransmission or synchronization of the
authentication state. In media such as IEEE 802.11, which provides
for retransmission, as well as synchronization of authentication
state via the 4-way handshake defined in [IEEE-802.11i], additional
resilience is typically of marginal benefit.
Depending on the method and circumstances, result indications can be
spoofable by an attacker. A method is said to provide protected
result indications if it supports result indications, as well as the
"integrity protection" and "replay protection" claims. A method
supporting protected result indications MUST indicate which result
indications are protected, and which are not.
Protected result indications are not required to protect against
rogue authenticators. Within a mutually authenticating method,
requiring that the server authenticate to the peer before the peer
will accept a Success packet prevents an attacker from acting as a
rogue authenticator.
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However, it is possible for an attacker to forge a Success packet
after the server has authenticated to the peer, but before the peer
has authenticated to the server. If the peer were to accept the
forged Success packet and attempt to access the network when it had
not yet successfully authenticated to the server, a denial of service
attack could be mounted against the peer. After such an attack, if
the lower layer supports failure indications, the authenticator can
synchronize state with the peer by providing a lower layer failure
indication. See Section 7.12 for details.
If a server were to authenticate the peer and send a Success packet
prior to determining whether the peer has authenticated the
authenticator, an idle timeout can occur if the authenticator is not
authenticated by the peer. Where supported by the lower layer, an
authenticator sensing the absence of the peer can free resources.
In a method supporting result indications, a peer that has
authenticated the server does not consider the authentication
successful until it receives an indication that the server
successfully authenticated it. Similarly, a server that has
successfully authenticated the peer does not consider the
authentication successful until it receives an indication that the
peer has authenticated the server.
In order to avoid synchronization problems, prior to sending a
success result indication, it is desirable for the sender to verify
that sufficient authorization exists for granting access, though, as
discussed below, this is not always possible.
While result indications may enable synchronization of the
authentication result between the peer and server, this does not
guarantee that the peer and authenticator will be synchronized in
terms of their authorization or that timeouts will not occur. For
example, the EAP server may not be aware of an authorization decision
made by a AAA proxy; the AAA server may check authorization only
after authentication has completed successfully, to discover that
authorization cannot be granted, or the AAA server may grant access
but the authenticator may be unable to provide it due to a temporary
lack of resources. In these situations, synchronization may only be
achieved via lower layer result indications.
Success indications may be explicit or implicit. For example, where
a method supports error messages, an implicit success indication may
be defined as the reception of a specific message without a preceding
error message. Failures are typically indicated explicitly. As
described in Section 4.2, a peer silently discards a Failure packet
received at a point where the method does not explicitly permit this
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to be sent. For example, a method providing its own error messages
might require the peer to receive an error message prior to accepting
a Failure packet.
Per-packet authentication, integrity, and replay protection of result
indications protects against spoofing. Since protected result
indications require use of a key for per-packet authentication and
integrity protection, methods supporting protected result indications
MUST also support the "key derivation", "mutual authentication",
"integrity protection", and "replay protection" claims.
Protected result indications address some denial-of-service
vulnerabilities due to spoofing of Success and Failure packets,
though not all. EAP methods can typically provide protected result
indications only in some circumstances. For example, errors can
occur prior to key derivation, and so it may not be possible to
protect all failure indications. It is also possible that result
indications may not be supported in both directions or that
synchronization may not be achieved in all modes of operation.
For example, within EAP-TLS [RFC2716], in the client authentication
handshake, the server authenticates the peer, but does not receive a
protected indication of whether the peer has authenticated it. In
contrast, the peer authenticates the server and is aware of whether
the server has authenticated it. In the session resumption
handshake, the peer authenticates the server, but does not receive a
protected indication of whether the server has authenticated it. In
this mode, the server authenticates the peer and is aware of whether
the peer has authenticated it.
8. Acknowledgements
This protocol derives much of its inspiration from Dave Carrel's AHA
document, as well as the PPP CHAP protocol [RFC1994]. Valuable
feedback was provided by Yoshihiro Ohba of Toshiba America Research,
Jari Arkko of Ericsson, Sachin Seth of Microsoft, Glen Zorn of Cisco
Systems, Jesse Walker of Intel, Bill Arbaugh, Nick Petroni and Bryan
Payne of the University of Maryland, Steve Bellovin of AT&T Research,
Paul Funk of Funk Software, Pasi Eronen of Nokia, Joseph Salowey of
Cisco, Paul Congdon of HP, and members of the EAP working group.
The use of Security Claims sections for EAP methods, as required by
Section 7.2 and specified for each EAP method described in this
document, was inspired by Glen Zorn through [EAP-EVAL].
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9. References
9.1. Normative References
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC1994] Simpson, W., "PPP Challenge Handshake
Authentication Protocol (CHAP)", RFC 1994, August
1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC2243] Metz, C., "OTP Extended Responses", RFC 2243,
November 1997.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", RFC 2279, January 1998.
[RFC2289] Haller, N., Metz, C., Nesser, P. and M. Straw, "A
One-Time Password System", RFC 2289, February
1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 2434, October 1998.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's
Retransmission Timer", RFC 2988, November 2000.
[IEEE-802] Institute of Electrical and Electronics Engineers,
"Local and Metropolitan Area Networks: Overview
and Architecture", IEEE Standard 802, 1990.
[IEEE-802.1X] Institute of Electrical and Electronics Engineers,
"Local and Metropolitan Area Networks: Port-Based
Network Access Control", IEEE Standard 802.1X,
September 2001.
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9.2. Informative References
[RFC793] Postel, J., "Transmission Control Protocol", STD
7, RFC 793, September 1981.
[RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September
1993.
[RFC1750] Eastlake, D., Crocker, S. and J. Schiller,
"Randomness Recommendations for Security", RFC
1750, December 1994.
[RFC2246] Dierks, T., Allen, C., Treese, W., Karlton, P.,
Freier, A. and P. Kocher, "The TLS Protocol
Version 1.0", RFC 2246, January 1999.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March
1998.
[RFC2486] Aboba, B. and M. Beadles, "The Network Access
Identifier", RFC 2486, January 1999.
[RFC2408] Maughan, D., Schneider, M. and M. Schertler,
"Internet Security Association and Key Management
Protocol (ISAKMP)", RFC 2408, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key
Exchange (IKE)", RFC 2409, November 1998.
[RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP
Extensions", RFC 2433, October 1998.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and
Policy Implementation in Roaming", RFC 2607, June
1999.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G.,
Zorn, G. and B. Palter, "Layer Two Tunneling
Protocol "L2TP"", RFC 2661, August 1999.
[RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS
Authentication Protocol", RFC 2716, October 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W.
Simpson, "Remote Authentication Dial In User
Service (RADIUS)", RFC 2865, June 2000.
Aboba, et al. Standards Track [Page 60]
RFC 3748 EAP June 2004
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla,
M., Zhang, L. and V. Paxson, "Stream Control
Transmission Protocol", RFC 2960, October 2000.
[RFC3162] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and
IPv6", RFC 3162, August 2001.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC
3454, December 2002.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote
Authentication Dial In User Service) Support For
Extensible Authentication Protocol (EAP)", RFC
3579, September 2003.
[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G. and J.
Roese, "IEEE 802.1X Remote Authentication Dial In
User Service (RADIUS) Usage Guidelines", RFC 3580,
September 2003.
[RFC3692] Narten, T., "Assigning Experimental and Testing
Numbers Considered Useful", BCP 82, RFC 3692,
January 2004.
[DECEPTION] Slatalla, M. and J. Quittner, "Masters of
Deception", Harper-Collins, New York, 1995.
[KRBATTACK] Wu, T., "A Real-World Analysis of Kerberos
Password Security", Proceedings of the 1999 ISOC
Network and Distributed System Security Symposium,
http://www.isoc.org/isoc/conferences/ndss/99/
proceedings/papers/wu.pdf.
[KRBLIM] Bellovin, S. and M. Merrit, "Limitations of the
Kerberos authentication system", Proceedings of
the 1991 Winter USENIX Conference, pp. 253-267,
1991.
[KERB4WEAK] Dole, B., Lodin, S. and E. Spafford, "Misplaced
trust: Kerberos 4 session keys", Proceedings of
the Internet Society Network and Distributed
System Security Symposium, pp. 60-70, March 1997.
Aboba, et al. Standards Track [Page 61]
RFC 3748 EAP June 2004
[PIC] Aboba, B., Krawczyk, H. and Y. Sheffer, "PIC, A
Pre-IKE Credential Provisioning Protocol", Work in
Progress, October 2002.
[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2)
Protocol", Work in Progress, January 2004.
[PPTPv1] Schneier, B. and Mudge, "Cryptanalysis of
Microsoft's Point-to- Point Tunneling Protocol",
Proceedings of the 5th ACM Conference on
Communications and Computer Security, ACM Press,
November 1998.
[IEEE-802.11] Institute of Electrical and Electronics Engineers,
"Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications", IEEE
Standard 802.11, 1999.
[SILVERMAN] Silverman, Robert D., "A Cost-Based Security
Analysis of Symmetric and Asymmetric Key Lengths",
RSA Laboratories Bulletin 13, April 2000 (Revised
November 2001),
http://www.rsasecurity.com/rsalabs/bulletins/
bulletin13.html.
[KEYFRAME] Aboba, B., "EAP Key Management Framework", Work in
Progress, October 2003.
[SASLPREP] Zeilenga, K., "SASLprep: Stringprep profile for
user names and passwords", Work in Progress, March
2004.
[IEEE-802.11i] Institute of Electrical and Electronics Engineers,
"Unapproved Draft Supplement to Standard for
Telecommunications and Information Exchange
Between Systems - LAN/MAN Specific Requirements -
Part 11: Wireless LAN Medium Access Control (MAC)
and Physical Layer (PHY) Specifications:
Specification for Enhanced Security", IEEE Draft
802.11i (work in progress), 2003.
[DIAM-EAP] Eronen, P., Hiller, T. and G. Zorn, "Diameter
Extensible Authentication Protocol (EAP)
Application", Work in Progress, February 2004.
[EAP-EVAL] Zorn, G., "Specifying Security Claims for EAP
Authentication Types", Work in Progress, October
2002.
Aboba, et al. Standards Track [Page 62]
RFC 3748 EAP June 2004
[BINDING] Puthenkulam, J., "The Compound Authentication
Binding Problem", Work in Progress, October 2003.
[MITM] Asokan, N., Niemi, V. and K. Nyberg, "Man-in-the-
Middle in Tunneled Authentication Protocols", IACR
ePrint Archive Report 2002/163, October 2002,
<http://eprint.iacr.org/2002/163>.
[IEEE-802.11i-req] Stanley, D., "EAP Method Requirements for Wireless
LANs", Work in Progress, February 2004.
[PPTPv2] Schneier, B. and Mudge, "Cryptanalysis of
Microsoft's PPTP Authentication Extensions (MS-
CHAPv2)", CQRE 99, Springer-Verlag, 1999, pp.
192-203.
Aboba, et al. Standards Track [Page 63]
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Appendix A. Changes from RFC 2284
This section lists the major changes between [RFC2284] and this
document. Minor changes, including style, grammar, spelling, and
editorial changes are not mentioned here.
o The Terminology section (Section 1.2) has been expanded, defining
more concepts and giving more exact definitions.
o The concepts of Mutual Authentication, Key Derivation, and Result
Indications are introduced and discussed throughout the document
where appropriate.
o In Section 2, it is explicitly specified that more than one
exchange of Request and Response packets may occur as part of the
EAP authentication exchange. How this may be used and how it may
not be used is specified in detail in Section 2.1.
o Also in Section 2, some requirements have been made explicit for
the authenticator when acting in pass-through mode.
o An EAP multiplexing model (Section 2.2) has been added to
illustrate a typical implementation of EAP. There is no
requirement that an implementation conform to this model, as long
as the on-the-wire behavior is consistent with it.
o As EAP is now in use with a variety of lower layers, not just PPP
for which it was first designed, Section 3 on lower layer behavior
has been added.
o In the description of the EAP Request and Response interaction
(Section 4.1), both the behavior on receiving duplicate requests,
and when packets should be silently discarded has been more
exactly specified. The implementation notes in this section have
been substantially expanded.
o In Section 4.2, it has been clarified that Success and Failure
packets must not contain additional data, and the implementation
note has been expanded. A subsection giving requirements on
processing of success and failure packets has been added.
o Section 5 on EAP Request/Response Types lists two new Type values:
the Expanded Type (Section 5.7), which is used to expand the Type
value number space, and the Experimental Type. In the Expanded
Type number space, the new Expanded Nak (Section 5.3.2) Type has
been added. Clarifications have been made in the description of
most of the existing Types. Security claims summaries have been
added for authentication methods.
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o In Sections 5, 5.1, and 5.2, a requirement has been added such
that fields with displayable messages should contain UTF-8 encoded
ISO 10646 characters.
o It is now required in Section 5.1 that if the Type-Data field of
an Identity Request contains a NUL-character, only the part before
the null is displayed. RFC 2284 prohibits the null termination of
the Type-Data field of Identity messages. This rule has been
relaxed for Identity Request messages and the Identity Request
Type-Data field may now be null terminated.
o In Section 5.5, support for OTP Extended Responses [RFC2243] has
been added to EAP OTP.
o An IANA Considerations section (Section 6) has been added, giving
registration policies for the numbering spaces defined for EAP.
o The Security Considerations (Section 7) have been greatly
expanded, giving a much more comprehensive coverage of possible
threats and other security considerations.
o In Section 7.5, text has been added on method-specific behavior,
providing guidance on how EAP method-specific integrity checks
should be processed. Where possible, it is desirable for a
method-specific MIC to be computed over the entire EAP packet,
including the EAP layer header (Code, Identifier, Length) and EAP
method layer header (Type, Type-Data).
o In Section 7.14 the security risks involved in use of cleartext
passwords with EAP are described.
o In Section 7.15 text has been added relating to detection of rogue
NAS behavior.
Aboba, et al. Standards Track [Page 65]
RFC 3748 EAP June 2004
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1 425 706 6605
Fax: +1 425 936 6605
EMail: bernarda@microsoft.com
Larry J. Blunk
Merit Network, Inc
4251 Plymouth Rd., Suite 2000
Ann Arbor, MI 48105-2785
USA
Phone: +1 734-647-9563
Fax: +1 734-647-3185
EMail: ljb@merit.edu
John R. Vollbrecht
Vollbrecht Consulting LLC
9682 Alice Hill Drive
Dexter, MI 48130
USA
EMail: jrv@umich.edu
James Carlson
Sun Microsystems, Inc
1 Network Drive
Burlington, MA 01803-2757
USA
Phone: +1 781 442 2084
Fax: +1 781 442 1677
EMail: james.d.carlson@sun.com
Henrik Levkowetz
ipUnplugged AB
Arenavagen 33
Stockholm S-121 28
SWEDEN
Phone: +46 708 32 16 08
EMail: henrik@levkowetz.com
Aboba, et al. Standards Track [Page 66]
RFC 3748 EAP June 2004
Full Copyright Statement
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Aboba, et al. Standards Track [Page 67]